I don't know what's legal in the UK, but in the US -- as long as you don't need rider-to-passenger intercom, music, GPS, etc. all tied into the system -- the easiest and least expensive way to get bike-to-bike communications is to get Family Radio Service (FRS) transceivers. The Motorola Talkabout is one well known brand.
Some models require a Push-to-Talk (PTT) switch on the radio itself or on an extension cable to be pressed in order to talk, while others will work with a built-in Voice Activated Switch (VOX) controlled transmiter. Some models even use a bone-conduction microphone built into the earpiece rather than a seperate boom mic on a headset. Those are great for use on a bike; just put the earphone in your ear, put on your helmet, and you're ready to go.
More advanced (and more expensive) units operate on the General Mobile Radio Service (GMRS) band that allows more legal power for greater range, and the top of the line models will work on either band.
Again, I don't know what's legal in the UK, but a visit to an electronics store should get you the relevant information. No need to go to a motorcycle shop where the prices are likely to be higher for motorcycle-specific gear, when Radio Shack or equivelant will do for a start.
HTH.