To clarify
The employer can change the terms of employment to anything they like as long as they pay the legal minimum wage of £6.20 an hour. All they have to do is give the legal notice period of the new conditions and you either agree or have been deemed to have resigned
Sorry, hard but true. I have not heard of a paid lunch break for over 20 years now !
To answer your question, they do not need to MAKE you sign a new contract they just have to give notice and you either agree or leave , simple.