Twostep, you raise in interesting issue here! I was, of course, thinking of the UK tax folks talking to the UK benefits folks. But what about the US tax folks talking to the UK tax folks? That one is actually likely.

Technically, the family allowance would have to be reported on the US tax returns (because in the US you're taxed on worldwide income). If you don't report it, it can come back to bite you forever (i.e. no statute of limitations on income not reported).

I don't know how it's treated in the UK, but I have heard some reports of the US cross checking tax returns with other countries.

Oh well, I guess we'll hear about it if it doesn't work out.
Cheers,
Bev