There are a few options you could look at.
CB is a good option when travelling with others, I have justed fitted a £60 rig which I bought from Armada, with a £25.00 magmount aerial. I use this for 4x4 Response stuff more than anything, there is zero use of the cb frequencies around here, so the old problem of idiots has gone.
It gives a range of a few miles.
Handheld CB's seem to start at about £110 new, in an ideal world you would have one of each for comms with your passenger when they are out on foot. If you buy one, make sure it has all the UK and EU channels, new ones do, secondhand may not.
The alternative could be PMR446 radios, these start at about £20 a pair upwards, second hand Kenwood TK3101's are an excellent buy, about £40 each. There is no licence needed.
They give a range of maybe 1000m, are ideal for close stuff, but struggle beyond that.
You could sit the Foundation radio licence course, which would allow you to use a 2m or 70cm radio, the 2m band can use repeaters, so a range of 50 - 60 miles is achievable - but....this wouldnt work abroad as the licence is only for UK use.
I have heard that some countries do not allow radio comms. although I dont know how much of an issue it is.
Regards
Pete