2 way radio for expedition groups


I’m a teacher who supervises groups of students on expedition. I used to use a gps tracker to keep track of them but they’re really temperamental so I’m looking for a walkie talkie setup instead.

It would need to be something available in the UK, have a range of at least a few miles (actual range, rather than manufacturers claimed range) be shower proof and have a decent battery life, at least a few days, but won’t be used that much.

What would you recommend? Budget of around ?100 for three handsets.