I think you may have dreamt that one up VS, there are no 747's based in LGW, the crew are only trained on the 777, 737 and Airbus.
But a "w" pattern could easily be rostered, say the 777 has 10 crew and the A318 has 4. 10 crew would operate LGW-JFK on day one. On day two 4 of those crew on their own itinerary operate NYC-LCY while the other 6 are joined by the 4 crew from the previous days LCY-NYC. Day three... land in LCY. Day four the four crew go LCY/SNN-NYC and then on day 5 they join the other 6 crew for the 777 flight back to LGW. Flights all fully crewed, no costly transport costs within london, full utilisation of cabin crew. However obviously hotel costs in London need to be factored in but there is no reason why they could not actually rest in SNN instead at a cheaper cost. However I personally would hate to have to hang around london to bring fresh passengers to SNN after doing a night flight in from NYC.
Even with 2 LCY flights a day this could be easily achieved. Simply the Cabin Manager and another main crew member remain on the JFK round trip with the rest of the crew splitting to operate the LCY services based on the assumption that the A318 will only have a purser and three main crew!
At LHR we do a lot of seperate itinerary trips from the crew we initially report with. For example I did a B2B recently which was an IAD/BWI trip. I flew out to IAD with 12 crew in a 777. The next day we were met by 3 more crew and we flew home on a 747. Then the next day we lost 7 crew and only 8 of us flew to BWI and back on a 767!! Almost every trip we had crew joining or leaving the core 8 of us!