To a degree it depends on the expectation - and how reasonable that expectation is - that an airline will have spare crew hanging about at every outstation or how easy it is to get one to an airport from another one and to a degree that depends on the airports involved.
As c-w-s says at LHR and LGW you can more than reasonably expect BA to have crew either there on standby or within a short distance away on call but they are main bases. Personally I'd say the same for JFK where BA has several flights a day but I know others might disagree with me on that.