Re: Common practice
I've seen it, too. Always refused to sign until removed but...
Since the terms were illegal under UK and (at the time) EU laws, maybe I should have signed and, if a later dispute occurred, I'd be home and dry, since illegal clauses would free me from any contractual obligations.
The thing to remember is, contract or not, nothing can take away your rights in law. I've won two legal disputes with former employers and one on behalf of my wife. OTOH, employment protection laws are being eroded, so what applied then may not apply now.