Matter of trust

ATM in London support cash withdrawal only with no way of replenishment, and you have to visit bank's branch for cashiers. Another way available is to use so-called "quick deposit envelopes":



Having an envelope, you can write your account or card number on the flap, put your cash in and drop it to a special box in the branch. Sometimes holes of those boxes are placed even outside, on a bank's building wall, so you can do that when the branch is closed.