IIRC last time I saw a Which report, the bigger the bank, the worse the service.
there was a time when service depended on how good your Branch Manager was. I think those days have gone now.
I imagine you will want overdraft facility, low charges, internet banking, linked deposit account that pays a good rate on the money you've tucked away to give to the taxman. Possibly also "Quickpay" facilty so you can pay things you have forgotten about and avoid late payment charges.