basically, as people above have indicated, you'll find the majority of pedals that use external power supplies will just be a 120vac (US mains) to 9vdc adaptor. In the UK you'll just need to buy a 240vac power supply - no big deal.
What you need to watch for are the pedals that have built-in power supplies like some valve overdrives. If the mains transformer is internal you will either need to source a replacement for 240v or (easier), get a step-down transformer so that the pedal gets the 120v it wants rather than the 240v we have here in the UK.