The question you asked is:
If I take redundancy on a contract that says there is an 8 wk notice period, can they say it is now 4 wks... ?
No is the short answer.
How long have you been employed with them ? If it is less than 24 mo I think you have no rights at all - they can just lay you off. If it is longer then the redundancy pay is one week for each month you have been employed.....with a cap of course.
before you ring ACAS which I think is excellent advice,
read this:
https://www.gov.uk/employer-relocation-your-rights