Hmm. There's a shed-load of misleading statements in the above, so I'm going to have to start again.
The starting point, on the basis that teaching a man/woman to fish (or where to fish) then you feed him/her for life is to read this document: -
http://www.direct.gov...onditions/DG_10028079
The most important bit is titled 'If your employer wants to change your contract' - read it carefully, then click onto the page where it talks about 'disagreeing to changes in your employment contract'.
You will note it says 'if you disagree the employer is not allowed to just bring in the change'. That's the good news and Maidup is correct about that. However the employer can terminate your contract by giving you minimum notice (probably two weeks in your case but more if your written contract says longer) and offering to re-engage you on revised terms (the changed hours). The DirectGov website tells you about contacting ACAS if this happens to you.
What the employer should be doing right now is consulting with you over the changes, finding out what practical problems it would cause you and offering to try and find ways around them. Equally it helps your case (if it comes to it down the line) if you are able to show SOME flexibility in recognising the employer's problem. Feel free to make your own suggestions (that don't necessarily involve you in agreeing the hours). KEEP NOTES OF THESE MEETINGS.
Then see what happens. If the employer still goes ahead and offers you only the changed hours scenario, you could go and talk to ACAS. This might then lead eventually to a claim for unfair dismissal. (Its 12 months continuous service before a claim for unfair dismissal can be made - not 24 months).
You do not have 'protected rights' in a way that they are sacrosanct and cannot be changed.