Not true: The regs simply state it must be able to be moved in two or three different places. Which you can still do with fixed buckets, it just means removing them and re-bolting them in a different place, so perfectly possible In reality it depends on the MOT tester, some are sensible and some are not. The MOT regs also make exception to things like removal of airbags along with the seat rules for significantly modified vehicles: It's aimed at the proper rally cars and road-legal race cars, but it can also happily take into account significantly modified regular road cars too.
This is how my old MR2 & MX5 passed MOT every year, when they were stripped out with fixed buckets.