Uk third party payment processor wanted

I need to be able to bill in GBP ,my current processors says they do this but my customers are being charged more than what is stated when they signup ,this is due to them being billed in a foreign currency and their bank charging them.
i don`t know why these places offer billing in GBP if it only works for some customers and not others.

Anyway does anyone know of a UK based company offering similar services as worldpay (not paystamp)
I don`t need to bill any other currency just GBP any help appreciated.