I like this problem because it seems to operate on the same intuitions that lead to one-boxing and two-boxing for those who don't do any actual analysis, but the one-boxing intuition leads you astray (though not by much).

Personally, I'd take the £10 on reflection but would have refused the £10 based on my intuitions. I'm pretty sure Omega wouldn't be giving me £10, since if confronted with the situation I would be forced to think, "If I say 'no' now, there's lots of money in that envelope."