I can't comment on your specific order as I don't have the details in-front of me, but basically our system works like this: Our system works by effectively buying-by-proxy - and its correct that we are not buying in BTC/GBP. We can't - only GOX has a GBP wallet and that through the Japanese bank deposit. So initially we fill our wallets with whatever the best value currency we can get is (EUR or USD typically) from our bank.  We then take GBP value of your deposit (less fee) - multiply it by whatever the exchange rate is for that currency (this is the exchange rate we got when we made our deposit - not the inter-bank lending rate) and create a BUY order for the resultant amount in the required currency at the target exchange. We only complete an order when we've consumed the full amount of fiat that we calculated the order was worth in the currency the exchange wallet is denominated in at whatever price is necessary to fulfill it. So technically, all or part of the order could (though unlikely) end up paying the exchange high price for that time period. When the exchange spits out a value to tell us how many coins we received for X fiat - we send you that value and record the details (which you can view in 'Recent Transactions' on our site).

We are designed to be a fast, reliable (hopefully) mechanism for purchasing BTC - but not necessarily the cheapest.