The UK splits the legal stuff into an offer letter (”principal statement”) and the fine print (”wider statement”.) The latter is what most people informally call the contract, including the front page of these documents.
Employers must to provide you with the fine print within two months of your start date:
Any incontrovertible evidence of an agreement of the job, verbal or written, counts as a contract. Everything else is just finalizing the terms and conditions, which either party can agree to amend at any later date and which many employers assert they can do unilaterally.
I'm guessing we might be in different sectors, but FWIW I've never seen this in my >20 years in the UK. To me the situation you're describing would ring all sorts of alarm bells.
P.S. Not doubting your experience, just comparing it with mine.
