I would suggest looking at either Affiliate window or affiliate future, they are quite big in the UK affiliate industry. In terms of startup costs it’s going to vary wildly on the network you choose (especially if you go with someone like CJ).
The other option is running your own affiliate program but this ofcourse bring in extra costs (hiring staff. promotion etc)
Checkout the forums on http://www.affiliates4u.com their are plenty of affiliate networks and experienced affiliate managers for mainstream sites on their.