There is no categoric answer to your dilemma.
If you refuse to sign, an employer
might terminate your contract. If they do, there's a good chance a tribunal would deem it unfair, but it is
not a certainty.
A lot will depend on what the variations in the contract are about (i.e. what the old one said and what the new one says). It would also depend on
why the employer wanted to change it. It might be, for instance, that there is a very good business reason for making a change, and that it could be something that
if it were done capriciously or without good reason, would result in a tribunal deciding that terminating the contract was unfair dismissal, but if the company can justify to the tribunal why the change was necessary, then they would rule it fair.
From what I'm being told, there's a good chance such a case would go in your favour, but it is
not a certainty.
Oh, and if any future employer finds out you took the last one to a tribunal, well, fair or not, that WILL cost you a job offer with some firms. I state that as a fact, because I know of firms that have done it. Their attitude is .... "why risk it? He might have had a good reason,but might just be a trouble-maker". And at that point, you get binned. It's not fair, and may not be legal .... but try proving it.
So .... do you have to sign it? No, as far as I'm aware, you don't.
But, contract changes can also become effective if the firm seek to put it in place and you work under the new terms without objection. So if you are really determined to not agree, either put it in writing, or cross out the bits of the new contract you don't agree to, sign it and then for pities sake, KEEP COPIES.
But .... if you don't sign it, you
might end up fired, you
might lose any industrial tribubal, you
might end up marked as a troublemaker even if you aren't fired, and you
might be putting future employment opportunities (and references) at risk.
So, decide if it's a fight you're prepared to pick. Is the issue important enough?
Oh, and I've been in almost exactly that situation, and refused to sign. The differences with me were that the implications of the changes were potentially vastly more significant, and I would NOT have carried on working under the terms terms. I'd have walked. And, it was a contract for ad-hoc contract work, not employment, so a tribunal wasn't an option anyway.
One last point. All the above is personal OPINION, not advice. I'm not qualified to give legal advice .... and if I was, you'd be getting a nice, large bill attached.
But I'll give one free piece of advice .... if you want legal advice you can rely on, get it from a qualified lawyer, and preferably one with an employment law specialisation, not from a forum.
I know you're after opinions, not advice here, but just the same, do NOT treat my opinion or anyone else's here as gospel when your job and prospects, especially in the current climate, may depend on it.