The only time I have needed to offload fuel was when I hired a Piper Arrow at Madison in the USA and had a very oversized passenger. I asked the renting company and they said "no-problem". It wasn't either... the fuel truck pulled up, a special nozzle was put into the tanks and I was asked how much avgas I wanted offloaded. I seemed like a routine job to them and took all of a couple of minutes.
Don't ask me if the avgas went into a separate tank on the fuel truck because I simply don't know, but the speed and efficiency suggested it was a routine operation. I've never seen it done in the UK, although to be fair I have never needed to ask.