Hi!
I'm Gaurav and I have been integrating payment gateways with custom websites for over a year now.
Recently, I worked with a US client to integrate Stripe on his site.
I checked WorldPay API (haven't worked with WorldPay earlier, frankly!) and there's an API available for PHP (my language of choice) as well as for major frameworks - Ruby, Perl, anything!
I think it shouldn't be very tough to do what you require.
Hence, I am quoting 100 Pounds.
I will need to have your client key using which I can access both a live and sandboxed version of WorldPay and then I can build the payment system on top of it.
Ask me any questions if you have!