I am also in the UK and have a fully US Adwords account, I just entered a US address and then used my UK credit card info, no problem. You can also avoid paying the UK VAT this way too, and the bid increments are smaller at 1 US cent, rather than 1 UK p, so effectively you can outbid someone by bidding helf a pence more, which adds up over the thousands of clicks.
Even though im in the UK, i use the dollar to set up my account...