The system is called PMR446 (I've corrected the thread title) and you should be OK purchasing PMR446 handsets from any reputable dealer in the UK. To be legal, the equipment must be manufactured specifically for use on the PMR446 system - you can't just tune an old amateur radio onto the same frequencies.
You'll find them at Maplin, pretty much any amateur radio vendor, main supermarkets and toy shops. Be wary of cheapo stuff from the Internet which may operate on the same frequencies but might not actually comply with the proper spec and regulations.
Be aware that anybody can buy and use this kit and MOD voice procedures aren't to be used with them.
Footnote: "PMR" is just an abbreviation for Personal (or Professional, or Private) Mobile Radio. "PMR446" is the specific system of license-free, low-power radios.