An airport that is often overlooked is Bristol, perhaps unsurprisingly given its modest size site nestling sleepily amongst the fields in the agreeable West Country.
BRS has had an easyJet Gibraltar service since 2015 that operates year round at mainly 3 x, sometimes 2 x, weekly. In the immediate pre-pandemic years it carried 48,000 to 50,000 passengers annually which, given the service frequency, seems to equate to decent loadings. We can't know the yield of course, but one might be justified in concluding that if it was dire the airline would not have retained the route for the past seven years. It's due to increase from its current 2 x weekly to 3 at the end of this month, thereafter the booking engine is showing 3 x weekly, except April which is 2, until the end of the summer timetable period. The BRS experience might suggest that it is possible to operate to Gibraltar from an airport that doesn't serve a major metropolitan area.
CAA stats show that in the calendar year of 2019 the annual passenger totals on UK-Gibraltar routes were:
Gatwick 193,000
Heathrow 163,000
Manchester 51,000
Bristol 50,000
Luton 26,000