Just to add some thoughts to this.
The primary locations for English qualified lawyers to move to are the Middle East (mainly Dubai and Abu Dhabi, although Saudi is growing), Singapore and HK (although less so the latter now) and Australia.
Generally, you are likely to continue practising English law in the Middle East, Singapore and HK rather than re-qualify. Those jurisdictions typically have large international transactions, for local clients, but governed by English law. A massive infrastructure project for the Saudi government, for example, will involve construction and finance agreements, all likely governed by English law. Likewise, a multi-ship finance deal in Singapore might also be governed by English law necessitating local English law capability. A mining project in Nigeria, led by French banks and sponsors, may need English project finance lawyers in Paris given the Anglophone nature of the Nigerian legal system.
Australia is different. I spent 10 years working there - initially as an English qualified lawyer, but then requalified in NSW which is a relatively easy process. I was with a major US/UK firm and then in-house. Australian law, and the local legal system, is very similar to that here and so the transition can be straightforward. Australian firms used to be very keen on English junior and mid-level associates given the quality of their training and experience: plus, they saw them as a useful resource who would come out for a few years and then return to the UK. To an extent, that route is a little less common now - mainly as a result of the mergers between Australian and UK firms (HSF, Ashurst, NRF etc) which enables those firms to move associates between jurisdictions, but also because of the tightening of the visa system and a glut of Aussies in London looking to return home. NZ is also an option, but the market there is tiny.
As Stiffy notes, there are also offshore opportunities (eg Caymans) and near-shore (eg Jersey) but those are limited and, potentially, career limiting given the nature of the work. I also know English-qualified lawyers working in Paris: predominantly in asset finance and project finance on Anglo related deals as mentioned above. The rest of the Continent is far harder to find opportunities, unless you are able to requalify, given the civil legal systems and the fact that deals in Germany, Spain and Italy tend to be more domestically focused.
The US is very difficult. There are some English lawyers there who continue to practise English law but usually where there is a significant nexus between the US, the UK and their specific practice area - perhaps UK-related securitisation matters for US banks for a US firm that doesn't have a London presence. We're talking about a small number of people. Lots of undergrads, with stars (or dollar signs) in their eyes, say they're going to move to the US but the market there is phenomenally competitive and the lack of local qualification will be a barrier to entry unless you're in one of those niche practice areas referenced above.
It is relatively common for trainees and associates at the larger firms to move overseas on secondment (eg for 6 to 12 months). The scope for that is largely dependent on practice area: transactional practices (banking, corporate) are easier, disputes-related practices are far harder given local Court procedures are usually very different to here. For most English qualified lawyers that secondment experience can "scratch the itch" of wanting to live overseas. It gets significantly harder to move overseas the more senior you get. As you approach partnership, it's all about demonstrating that you have your own practice and clients. Moving overseas for a significant period can disrupt the development of that business case for many.
Happy to share further thoughts.