Welcome to AW, sfogle!
I think your question is both clear and pertinent. The VAT rate does indeed affect writers querying agents in the UK, unless they are already registered for VAT themselves - which a US writer is unlikely to be.
VAT is added to an agent's commission. If they charge 15% of a writer's advance and royalties, the amount deducted by a UK agent will actually be 15% plus VAT (ie a further 17.5% of their percentage). The amount is unlikely to be big, but it should still be taken into account by writers comparing the rates of different agents in the US and UK.
There are other factors, of course, as Theo has already pointed out. Viability in the UK market is a crucial one, but you also need to consider how much harder it is to get an agent here in the first place. The
odds of landing a UK agent are around .1% - although if you
do get one then your odds of getting published are anything up to 90%. I don't know the latest figures in the US (and would be grateful if anyone knows a good link to them), but I've been told there's a better chance of getting an agent there in the first place, though many agents then offer only a 50-60% chance of publication.
You will also, as bertrigby has pointed out, need to check submission guidelines, as not all UK agents will consider overseas clients. This is a small country, and many of the big agents insist on face-to-face meetings with clients before signing them, which is bound to militate against those outside the UK. I believe there are exceptions, but you'll probably have the best chance with smaller, newer agents who are happy to work via e-mail.
Good luck either way - and again, welcome to AW.
Louise
ETA: The figures on getting agents in the UK are based on those published by the SofA three years ago, and may of course have changed (for the worse!) in the recent climate. However, the site I've linked to has very strong current links with UK agents and would probably have updated their figures if there were any significant change.