You might want to check out the TUPE (transfer of undertaking to protect employees rights, or something along those lines!) regulations. They should cover when you company was taken over but in essence they mean that existing employees contracts and terms are protected when the company they...
My advice would be to take some proper legal advice, possible from Citizens advice. There is another option and that is to write saying you don't accept the new terms and then leave. You would then be able to take them to a tribunal for constructive dismissal. However, get the legal advice...