I doubt it will happen.
Mobility to better "situations" (for want of a better word, I think in the US your net pay is more both in real and nominal terms, although only two weeks of holidays and up to 4 planes a week, think about it!) is VERY hard to achieve.
There might be quite a few consultants and senior consultants from the US/Canada. It doesn't mean there will be as many UK ones in the US or Canada, if any at all. In fact, US grads who know they wouldn't make it past the selection process in the US (where I trust Deloitte consulting does also more "prestigious" work and must have a serious startegy practice) often apply to the UK office to get a foot in the door, but forget about the other way round.
No matter what they advertise, these transfer always go downstream, from the more desirable location to the least desirable (it kinda follows how hard they work, too, sure in the US they are totally overworked on average).
So in North America you probably have carte blanche, in the UK you can probably transfer to Switzerland, China, maybe Australia if you have the nationality and probably mainland Europe if you have the language.
Big4 (and Deloitte maybe more than the other) do like luring people with the promise of great inter-functional and international transfers and flexibility, but reality is different.
On the other hand, it won't come down to a score or a ranking, so things could be moving your way if you know how to suck up to somebody, a key skill in that type of organisation...