In theory, your employer can't change your contract without your approval. unless its for the better (like a payrise, for example) and trying to enforce any changes would be a breach of contract.
In practice however, if there is an economic business reason meaning the company needs to make these changes, if you don't accept them (which you are entitled not to), you could be made redundant.
As to doing these things, if your organisation has more than 100 employees, you have a legal right to be consulted under the ICE regs before any changes are made. From April 08, that will extend to organisations of 50 or more employees. If you work for a company with fewer employees, you don't have to be consulted but for good employee relations, its recommended that you are.
Talk to your bosses about it