Agree, you will have to work with a local UK network althought a few US network are implementing themselves in the UK.
Tradedoubler, Buy.at, CJ, Affiliatewindow, AffiliateFuture, Webgains, Cleverat, Paidonresults, Afform, Silvertap, DGM, OMG, Zmirage etc, I count almost 15 affiliate networks in the UK. Any other I forgot ?

The choice depends notably on what the merchant activity is and what kind of commission model you will run.
If you are a merchant with a large product feed, you will need to optimize your product feed with a network that can support this, e.g. Tradedoubler.
If you are in a very specific industry there are a few second tier networks that have more potential than the big names because of their current merchants.
If you are planning to run an hybrid commission campaign and categorize your affiliate according to their conversion rates, you will have to work with networks having an interface allowing this.
If you have large ambitions and plan to open with more than one network, you will have to plan your negotiation carefully (negotiate first with a small network? one of the leaders? will they accept my campaign if I launch it with them afterwards ?)
If ...if... if... the best is that you contact an expert in Affiliate Management that knows about this market : me
Well Geno, we already know each other and I could have sent you an email, I'm just advertising for others
