Nextag UK, Shopping UK, Pricegrabber UK, PriceRunner, Ciao, and Kelkoo are some of the major comparison shopping players in the UK. If you are a US merchant looking to expand to comparison shopping sites internationally I'd first look at the Canadian shopping engines, they have much better conversion rates.
It's difficult to measure what comparison shopping engines are best for a merchant without knowing your product mix. An electronics merchant is better on one engine than a home & garden merchant, etc. I'd say all of those engines are manageable and worth trying, and sem4u presented one that should always be first (where applicable), Google Product Search.
Shopping comparison websites aren't all that popular in the UK. Shopping.com is The most popular of all I'd say, along with google product search. I don't think I've ever used a shopping comparison website other than google.