The instructions for Form 1116 should be the starting point for answering your questions. I don't believe the official USD-GBP and GBP-USD exchange rates differ enough to matter, since the official rates don't include fees for conversion. In any case, I would be using the GBP>USD exchange rate as the investment in GBP is fixed and known and you're trying to determine the USD equivalent of the amount.

FWIW I have a similar issue, because I have a brokerage account in Euro and occasionally buy/sell stocks from that.

I use the official daily exchange rates from the Federal Reserve for the calculations of my USD cost basis on the day of purchase and my USD sales price (less commission) on the day of sale: http://www.federalreserve.gov/releases/h10/Hist/

I hardly every transfer money into this account or from that account, and so far I have ignored those wire transfers between USD accounts and this Euro account for tax reporting purposes and only focused on the purchase/sales events WITHIN the Euro account.

Best Regards,
Bernhard