My 2p's worth:
1. The cheap ****ty walkie talkies (licence-free, PMR, half watt) you can buy at Dixons etc are basically crap in terms of range, even the good ones.
2. If you want walkie-talkies then either get 4 watt CB handhelds (I highly recommend the aforementioned Intek H-520), or 5 watt handhelds such as the Mitex range from Maplin, which you need a licence for but it is only £75 for 5 years. However with any hand-held, if you don't have line-of-sight you are very limited in range. Vehicle to vehicle (faraday cage to faraday cage) even more so. A mile at best, unless you get external antennas.
3. If you want car-mounted, get something that does all the EU and UK channels, but more importantly, the antenna is all, when it comes to range. Central mounted on the roof is best and with an electrical earth, so mag-mounts are not as good (besides they won't stick to ali anyway
If you don't want a hole in your roof, a gutter mount is a good bet but bizarrely, if you put it on the left you will have much higher range rightwards, and vice versa. The ground plane affects the range, a lot. If you have no ground plane in a given direction, you will have very limited range in that direction. You can do twin antennae but that gets complicated.
4. Don't believe the bollocks you read about having to have exactly 18 foot of co-ax to connect your aerial. It's bollocks.