Here is a small list of payment processors which are available to you. Remember to agree to the method of payment with clients before the contract has started.
A UK-based payment service.
Used regularly by freelancers for large payments. It is one of the most secure way of handling transactions as they have more conditions for buyers and sellers.
Great reputation with the option of cashing out money using a prepaid card.
An Australian based payment service.
Formerly Moneybookers, Skrill is very popular amongst UK freelancers.
Low fees of only 2.75% per transaction. All customers get a free mobile card reader.
A payment service that was created specifically for developers.