Employment law UK , my contract changed and I am required to give 3 month notice . What can they do if i dont?

You should be happy that you have achieved a sufficiently high status with your employer to gain 3 months security. Financially they are unlikely to take any action if you leave without notice unless you go to one of their competitors. Also don't expect to get a favourable job reference from them.
If your new employer wants you to start immediately, tell them that you're on 3 months notice and would be breaking your present contract and see their reaction at your interview. If they insist you start now say "I'll get a poor reference"

Nothing- i have walked out on two jobs and not given notice- they can stop your pay and take anything paid in lieu- but they have to pay you for hours worked and holdiay pay etc.

They probably won't waste their time doing anything, especially as they wouldn't want to create bad feeling for their company with you going round telling everyone what pooheads they are. That's if they're sensible.

IN theory they could sue you for breach of contract but they would find ir difficult to prove any losses worth taking you to Court for. They could refuse to give you a reference and a telephone call to a prospective employer can do a lot of damage and be unprovable. I would stick to your original contract if you have not signed or otherwise agreed to the change to agree to the change and give the old period of notice. Basically sod it just go!

They probably wouldn't give you a reference. They have to pay you for the time you have worked, but they probably won't pursue any legal action, as employers generally don't. If they did, you would need a pretty good reason for breaking a contract.

you need to check if they have a clause in the contract to withold payment if the correct notice is not given. If there is a clause like that they can legally withold money - if not they can take civil action for breach of contract (which would cost them - depends if they are arses or not really). best would be to give correct notice if you can or speak to them to come to a mutual agreement to alter notice period.

If they change your contract and do not give you an option then you can go to a Tribunal. The company may have to pay severance pay and release you.

You need to contact the advice line at ACAS as soon as possible. 08457 47 47 47. Dont sign the contract yet, as you may sign away your right to appeal. I'm not sure if this is a breach of your stautory employment rights, but all the sources I have had a look at today seem to say the say the same thing about notice periods.
Notice to be given by an employee
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An employee is required to give his or her employer at least one week鈥檚 notice if employed continuously for one month or more by that employer. This minimum is unaffected by longer service.
Have a look at the PDF at the link below for more info.

Your Employers should have given you at least 28 days notice to make any ammendments or changes to your contract.

Unless that you agrees to the changes and agree that the changes can take effect sooner.

If they have failed to give you the required notice, you do not have to agree to the changes.

If you are not happy with the changes, then they should give you the 28 notice, and you couldn't do anything.