The normal "interchange fee" for a cash withdrawal at a "remote" (not located at a bank branch) ATM is 25.9p. It's slightly lower if the ATM is located at a bank branch, slightly higher if it's a "protected ATM". Obviously the bank will have some costs of it's own on top of that but with everything computerised I wouldn't expect them to be very high.
Some withdrawals though will be substantially more expensive. Some ATMs are part of "financial inclusion" programs that result in significant extra payments to the ATM operator. Cash withdrals at the post office apparently cost the bank about 95p each in late 2019.