>>> trouble is there are so few co-op bank branches
You're certainly right there, Lankeela. There's only one left in Norfolk. (69 London Street, Norwich). The bank's website seems determined not to provide a branch telephone number but 01603 766809 should get you through to them. (Otherwise try 0345 721 2212. The call rate will be the same).
If you're not in a hurry though (and getting into Norwich is awkward for you), it might be simpler to take out £500 per day from your nearest cash machine. Alternatively, ask your local Post Office what the maximum amount you can withdraw there is:
https://www.postoffice.co.uk/the-co-operative-bank