Do you know if your US credit card provider would accept UK payment and do the exchange on their end? Sometimes using your UK debit card on a foreign website will only result in a small foreign transaction fee on the UK end (shouldn't be more than a pound or two).
I've only done a few UK to US wire transfers, and both times I've done it directly through my bank. The fees you get charged are going to depend on your bank, so it's worth calling them and asking (note that you can call US 800 numbers on Skype for free). Paypal might charge fewer fees or offer a better rate - someone else here can probably confirm that. But I can confirm that the bank to bank wires are secure - I've never had a problem going that route.
I actually have a joint account with my mother so she can handle any finances for me in the US, and she has suggested I could write her a check - in pounds - and send it to her to be cashed into the US bank. She thinks this would avoid all charges on the UK side and only result in charges at the US bank (my US bank has minimal charges). We haven't tried this yet but possibly will in a few months' time when I need to transfer more money. If you have someone in the US who could deposit a check and then transfer the money to your account, that could be an option.
Otherwise, I'm also interested to hear some responses since I have to transfer money about once a year to pay off my student loans in the US.