If you've got a
current account with Santander (rather than a savings account) you can withdraw cash from it and pay cash into it at any Post Office branch. You can also pay cheques in at any Post Office if you've previously obtained some personalised paying-in slips and deposit envelopes from Santander:
https://www.postoffice.co.uk/branch-banking-services
(Click the 'Withdraw and Deposits' tab)
With the Bank of England base rate (upon which all UK banks base their own interest rates) currently at just 0.1% p.a., you're not going to find any savings account that will earn you very much money. If you're making regular savings though (rather than frequently nipping into a bank to make individual deposits or to take money out), I can't really see why you actually need to be near to a bank branch anyway. It would seem to make more sense to me to look for the best savings account to meet your requirements
https://www.moneysavingexpert.com/savings/savings-accounts-best-interest/
or for the best ISA
https://www.moneysavingexpert.com/savings/best-cash-isa/