@Nico Albrecht
The business can’t do what it has done, fact.
The OP can contact the employer and point out that their existing contract transferred under TUPE did not include a weeks work in hand condition and as such please pay the money withheld to them. And then if not paid in say 7 days contact ACAS. If you think a gentler approach should be made.
BUT ACAS is set up for exactly this sort of thing, having been an employer on the other end of ACAS they are not to be frightened of, IMO they are brilliant, very very helpful and will work to avoid something going to legal stages at all costs, it’s a conciliation service to the benefit of both sides. If after consolidation an agreement still can’t be reached THEN you decide if it’s worth going legal or not.
IMO the OP should contact ACAS immediately, it’s clear to me the employer is oblivious of the law, as such what other terms do they think no longer apply?
As for joining a union so they have a collective voice, that’s something else, future option. I’m not a great fan of joining unions myself and it might not be a possibility here but it could be an option