I've taken the points above and added my own to form the essay as I would write it - hope this helps? Points in bold - could you please give more information/evidence? I'm not entirely sure what is meant by them.
To what extent would you agree that the Suez Crisis formed a turning-point in British imperial control?
The Suez Canal Crisis can indeed be considered ‘a turning-point’ for a number of reasons. In the first instance, it can be said to have ‘morally bankrupted’ Britain and France in the eyes of much of the global community with the uncovering of the secretive Franco-Anglo-Israeli Sevres Protocol, in which the three countries agreed on a method of attack against Egypt. The unspoken aim of the pact is generally read as an attempt to overthrow the Egyptian President Nasser – who was, for a number of reasons, intensely unpopular within the three states involved – and to reassert control over the newly-nationalised Canal, using an Israeli military threat against Egypt as a pretext for Anglo-French occupation of the Suez region; it has been largely deemed an underhanded war-plan by both contemporaries and, later, historians.
It can be seen as having shattered the imperialist mentality of the Conservative government within Britain – it demonstrated a fundamental weakness and, therefore, an inability on the part of Britain to continue to dictate policy to colonial or semi-colonial areas and, moreover, to enforce such wishes through military aggression. In line with this, the Crisis also marked the onset of direct US intervention - who threatened to cease all loans to Britain, were they to refuse to back down and withdraw from the region. This action would have intensified Britain’s economic vulnerability, given the country's dependence on the support. However, US threats were arguably limited as Britain had the support of several other economies, willing to supply them with funds. Additionally, America was, perhaps, unlikely to actually withdraw their aforementioned financial support, since it was feared (rationally or not) that a weak Britain could potentially fall to the USSR.
Perhaps, then, the main importance of this was not whether or not America actually held as much control over Britain as it may first appear, but that Britain backed down, regardless. This wish to avoid risking antagonising America can be viewed as an acknowledgement on the part of Britain of a change in the balance of global power. Certainly, it displayed a very clear weakness and a decided change from the 'myth of British invincibility' which had dominated perceptions of Britain in the 19th Century.
However, the extent to which the Suez Crisis itself can be seen as the turning point in Britain's control over its empire is limited. The Second World War should also be seen as a major turning-point since it was Britain's severely depleted finances and struggling economy which provided motivation for decolonisation (look at India + possessions in Africa). It could be noted that Britain had engaged successfully with Egypt over the Canal once before in the 19th century, as part of the scramble for Africa. At the time, they had used "gun-boat diplomacy" (Gladstone) and had won - thus it would indicate that Britain after World War Two were not so strong - also the importance of American influence as the US emerged from Isolationism. Add to this the importance of a changing economic system – the rise of multinationalism can be seen as having formed a sort of ‘neo-imperialism’ while the introduction of aircraft travel/transport meant that the Suez Canal was symbolic of Britain's old power, rather than essential in the way it had once been.
If one is to attempt to define a specific 'turning-point', I may propose World War One in lieu of the Suez Canal Crisis - it was this, after all, which generated the start of Britain's financial problems (both directly (through the sheer cost of war), and indirectly (Britain gained German territories in the Middle East - i.e. Iraq and Palestine - which later proved troublesome (lots of internal conflict between tribes in Iraq and the beginnings of the Israeli-Palestinian conflict in Palestine) and thus resource-hungry and costly to police/hold down. Thus World war One can be seen in terms of short-term gain and long-term loss).
Rather than attributing economic issues solely to World War Two, then, I would insted note that the economic weakening of Britain as a consequence of World War One was masked until World War Two due to the temporary eclipse of Britain's rivals (the USA was isolationist, Germany was in the midst of its own economic crisis and Russia was focussed on the establishment of its own regime and on its purges). World War Two brought Britain's rivals into the global arena whilst placing extra strain on its already-weak economy, leading to the dependence on American money which has already been mentioned - and it can be seen, therefore, not so much as a turning-point in itself as an extension of a previous turning-point. Alternatively, one could argue that it was simply one further event in a series; that it was part of a gradual process leading to decolonisation.
This process may even be seen, to some extent, as inevitable. Some historians do argue that the problems created by World War One, combined with a changing global economic system - from secured inter-colonial trade to the emergence of multinationals - made the eventual loss of Britain's empire an unavoidable eventuality which was only hastened by World War Two.
World War One can certainly be seen as part of a turning-point within the various Nationalist movements - it was 'fuel on the fire' in India and it led to a desire for greater freedoms to be bestowed upon the Dominions. World War Two, meanwhile, saw a change in the relationship between Britain and its former Dominions - although officially, most ties had been severed through the 1931 Statute of Westminster, Australia and New Zealand in particular had relied on Britain for military support and thus had maintained a close relationship. Following the attacks on Pearl Harbour, however, and with the British navy and air force bound largely in North Africa while Australia's own were supporting them there, Australia and New Zealand began to look elsewhere for protection (geographically-speaking, they were dangerously close to Japan). This came, ultimately, from the USA - it provides yet another example of power passing from British to American hands.
Another aspect to consider with regards to decolonisation is the idea of Cold War concerns. Although America was largely a force for decolonisation, it did permit Britain to maintain a hold over a number of areas of its Empire, mostly within the Middle and Far East, for a time after World War Two. This was under the policy of trusteeship - America wanted Britain to strengthen these areas economically before releasing them, in order to prevent them falling to Communist control. It provides a possible counter-argument to the idea that World War One or World War Two was a turning-point which turned and from there Britain just decolonised - because Britain didn't, immediately. It also fleshes it out more as a gradual process spanning around thirty years, which responded to the demands of its global context, be that a change to the way trade is conducted, or the balance of power - you could link this to a point about the emergence of politics on a global scale, perhaps.
Then again, you could also use it to show the extent to which Britain relied on America, and how dramatically its free will had been curbed by the late 1940s - that it had to enter into policies dictated by and primarily in the interests of the US. You could argue that, although America was letting Britain retain part of its empire, this was on America's own terms - and so, as a country which was essentially anti-imperialist, Britain was ruling in borrowed time: decolonisation, a direct inevitability. The American response to British actions in Suez, then, can be viewed as an end to trusteeship - or at least, as a demonstration of the limitations of American acquiescence.