I'm an amateur radio operator, and have a bit of experience with different kinds of radios.
Your best bet for longer-distance work would be something in the VHF frequency range (136-174 MHZ in the USA).
Over here we have an unlicensed radio band called MURS (Multi Use Radio Service) that has 5 channels in the 150MHz area. I have no idea if GB has MURS or something like it available.
FRS and GMRS Radios run in the UHF Frequency range (440MHz+), and that is why they don't get great range. They are a tad better for working inside buildings with dense walls as the UHF radio waves will pass through walls better, but they still don't get the range you can get with a pair of VHF radios.
If that was a tad too in-depth / geeky, let me know and I can try to explain it further.