Wikimedia UK certainly doesn't have a veto power on editathon's in the UK, we do try to coordinate in order to minimise clashes, but all we can do there is to request that people not have multiple editathons in the same city on the same day. There are things that we have that make editathon's easier to do or more inclusive - trained trainers, spare laptops, mobile Wifi and a hearing loop for starters, though the price for that support is that we do need to collect data for metrics. When someone independently organises an event in the UK we are likely to get in touch and offer spare laptops and other support.
Spare laptops in particular are very frequently used at UK editathons, and I believe "bring a laptop or book one of ours" is a much more inclusive way to run an event. Than "bring a laptop", and when someone discovers that they don't have a power lead or they weren't given the password for the laptop they brought it is really nice to be able to just hand them a laptop. So I always try to have one more than was ordered.
Regards
Jonathan Cardy
GLAM Organiser Wikimedia UK
after that it started to become impossible to organize an editathon without first having an employee agreeing it
That seems... wrong.
For one, that experience may be WMUK's but it's certainly far from universal. WMCA organizes monthly editathons in Montréal, at the very least, at zero cost. (They are organized/moderated by volunteers and the venue is provided at no cost by the Bibilothèque et Archive Nationale du Québec). I've never attended, but I'm told that they are fairly popular and well-liked.
If WMUK /chose/ do have a more structured (and more expensive) framework to organize similar events themselves, it in no way prevents volunteers or other organisms to do so without a penny of Foundation (or chapter) funding.