Let me flag that I work for one of the following firms.
One of these might work for you.
Maseco, Tanager, London and Capital, Independence Wealth Management.
I'm not necessarily saying these firms are all good: I am saying these are the only ones I know of who can answer your specialised question. I am not saying they will offer a service which you will find to be value for money.
I am not sure you want an IFA. You are not asking for advice on buying a new (retail) product, and I can't think of any with the requisite knowledge. The alternatives above are asset managers who also offer UK US planning with tax competence and some of them offer an in house tax service.