Currently there are just 4 BTTF time machines for hire in the UK:
Tom, in Devon:
http://www.delorean-timemachine.co.uk/Brian, in Brighton area:
http://www.delorean-hire.com/Ollie, near Bristol:
http://www.deloreanhire.com/and the fourth is a man named Steve, based in Kent, and I do not recommend you deal with him.
There are lots of "normal" DeLoreans though, which may lower the price. And I know of at least 2 other cars that are currently being converted to BTTF replicas, but I don't know if they will be hiring them out or not? Presumably they will though.
This is the definitive list - you can quote me on that!
Obviously it's not going to be cheap if the wedding is on the island. There is only 1 DeLorean on the Isle of Wight (a "normal" one) - and I don't think he does hire.