People keep saying this, but I'm not at all convinced anyone commenting really understands UK employment law. It's really only after 2 years of working for the same company that you get any decent employment protection.
It takes a bit more legal work, but if within the first 2 years they don't want you, they can get rid of you. If you're within your first 3 months of probation, they can just extend it to 6 months and get rid of you.
The UK is not a great country these days to go "employment laws keep you safe". There are better ones.
If you don't want these rug pulls and employment laws to keep getting watered down, instead of asking about 4 day work weeks, start voting for candidates and parties that have better employment laws at the top of their policies.