For UK based UL certification I suggest you contact the UL office near Manchester, I worked with them a bit in my last job. I don't have their details any more but the UL Guildford office should be able to put you in touch with them. I say the Manchester office because that's where they're actually setting up a test lab, if they can ever get the staff and train them up. Guildford is just a "customer service" place (sales monkeys and "account managers" on phones whom we refused to talk to, not engineers).
The big problem then was resource shortages (delays) at the Manchester office, which was only set up 2 years? ago but they may be able to point you at 3rd parties whom they consider accredited. Don't hold your breath, UL don't practise the concept of reciprocity - other agencies should accept their certificates but their auditors won't let them accept other agencies' !
However the ETL (Intertek) and CSA marks are widely accepted across America (I think). Whether you go with ETL, UL, CSA or whoever is largely determined by your market. For example ETL may be happy to certify product X but if the customers for that market segment have a spec that says "must meet UL standard blah" you have wasted money and will get reduced sales. Intertek have a substantial presence in the UK and with the resources they command here, can probably offer a faster service than UL for you.