Underground and buses in London can be paid contactless. In fact, on London buses, it's pretty much the only way now. On the underground it's almost always the cheapest way. You can use a card in London taxis as well. An increasing number of buses outside London also accept contactless.

Some smaller merchants still get awkward about accepting cards for transactions less than £5, even though legally in the EU they're not allowed to discriminate by payment method any more.

British ATMs generally all dispense £10 and £20 notes.