Is it legal to use a US booking site for United Airlines

[dac123]

I have 8 travel electronic travel certificated priced in US dollars which has a sterling equivalent. Because of the shift in exchange rate if I book the 8 flights to Orlando on the UK site I would have extra to pay, which I would rather not do. When I go online onto united.com it defaults to the US dollar priced web site. On this site the travel certificates more than cover the cost with about $260 left over for future use, a total swing of about $400 between the 2 sites. I was told that if I try to use a UK credit card the US system will reject it because of the address. In this case I would have nothing to pay as the travel certificates would cover the cost and I should not need to enter the CC in payment details. The question is if the system accepts the booking would I be doing anything illegal.