I use comms a huge amount on my expeds, so both Land Rovers have HF, VHF, VHF airband and UHF (as well as satphone).
Generally we go with the two 4wds, and up to 9 'bikes.
In short, VHF will give you line of sight 1km per watt (safe planning) - your PMR are limited by law to 0.5W (ideal conditions will give you up to 2km or so)
UHF (what you have) gives you less range, but supposedly more 'punch' through buildings etc. Not much advantage in the desert!
HF (used NVIS) will give you almost guaranteed coverage out to 150-200 km, but is MUCH more expensive, bulky and requires a higher level of competence than the other options.
Key to all systems is your antenna - for useable 20km range, you are going to need a 1+m whip (regardless of frequency) - that's it.
Once you have accepted that, options:
Cheapest: VHF 'mobile' rig, VHF 50W running off bike battery.
Smallest: VHF handheld, run into 50W amp, running off bike battery - gives you option to use handheld only whilst close.
When the bikes set off, they have handhelds each, and within the group they have one VHF amp(100W - hey, why not!), telescopic (not suitable on the move) antenna and a HF Manpack for emergencies).
Feel free to email if you have further questions.
One last thing, import/use of all these options (once you pass 0.5W) requires a licence in nearly every country of the world.