I don't know if it's common in the UK, I would guess not, but we have had thousands of kilometers of 25kV lines in France for half a century now, and I have never heard of people being electrocuted because their balloons had touched the catenaries.
Quite a lot of 25kV AC overhead in the UK. I'm sure it is possible to create a dangerous situation with those and a helium balloon, but it seems to be very rare. I don't think this is a real problem.