Remember the Ivy League is *not* the best 8 universities in the USA, it's a collection of universities in the same geographical region who've bandied together to form an athletics league. For example, Stanford, UC Berkeley and CalTech aren't part of the Ivy League, but are at least as good as some of them. Even in the same region, MIT isn't admitted, due to it's non-exceptional prowess at sports, despite being the hardest college in the US to get into (and with an exceptional academic record).
If you wanted to do something like the Ivy League - take the best known region for universities, pick the universities that are richest, have the longest tradition, have their own sports leagues and do exceptionally well academically, you'd have to say Oxbridge and UoL - the Golden Triangle. However Imperial, like MIT, lacks the sporting ability, and LSE doesn't offer a full range of subjects (unlike the Ivies), and so you'd have to discount those, leaving you with Oxford, Cambridge and UCL as the three universities that make up something most like an Ivy League.
But really, the term is almost non-sensical. The colleges in the US, like the UK, are pretty much in tiers, though as with any tiered system, the boundaries are variable, and the 'bottom' in each group is probably similar standard to the top in the next group.
My take for US:
Harvard, Yale, Princeton, MIT, CalTech
Stanford, UC Berkeley, Columbia, Cornell, Chicago, UPenn
Brown, Dartmouth, and a host of other really good schools
etc.
My take for the UK:
Cambridge, Oxford, Imperial
LSE, UCL, Warwick
Durham, York, Nottingham, Bristol, Edinburgh, KCL, etc.
Remember such lists for the UK really don't matter, as it all depends on subject, whereas for the US, you can decide/change your major later, and take a range of courses, so the overall level of the university is far more important.