Michael,
I've seen a lot of clients where this happens. Sometimes, a tax break in the UK (like office in the home) has a completely different set of rules than the US. If H&R Block gives you an office in the home deduction, they're saying it was used exclusively for business. If you own your UK residence, that's the exact opposite of what you want to say. Saying it was exclusively business use means that you'll pay UK capital gains on the office part of the sale of the resident. Most people living in the UK say the home office is not exclusively business, and thus lose the US office in home deduction.
There are dozens of other examples, the crucial factor is you don't want to save $$ in the US at the expense of ££ in the UK.