UK Air Passenger Duty and the way fares are filed out of London still make this a specialist market. A “London” ticket that arrives at Stansted at 23:10 with a closed rail line is a quote we would rather not send without a warning.
Heathrow
Terminals 2, 3 and 5 handle the alliance traffic you expect (Star, Oneworld, SkyTeam respectively, with exceptions). Terminal 4 still sees a mix of long-haul. Connection times look generous on paper; they are not if you land T3 and leave T5 with checked bags. We will not build a 70-minute self-connect and call it a round trip.
Gatwick, Stansted, Luton, City
Gatwick is a real scheduled airport, not only low-cost. Stansted and Luton work for leisure if the hotel is north or east, or if you are going straight to a hire car. London City has a steep approach, a short runway, and cabin baggage habits that catch people who packed for Heathrow.
Open-jaw worth doing
Arrive Heathrow, leave Dublin or Amsterdam (or the reverse) is a common work pattern. That is a multi-city request, not two one-ways taped together in a browser.