It's quite simple really. You ll never make money into LHR with a 50 seat aircraft when they charge high fees.
It's obvious that LHR would never last. Loganair were basically sitting on the slots and were paying for the lower landing fees at LHR, any subsidy or lack of charge (no one honestly knows) from MME was making it just about viable from what can be seen, as soon as LHR put the prices up its was never going to last.
LHR could work but would need to be marketed well, on an aircraft that makes fares competitive ie 130+ seat, at least twice a day but more realistically 3 times a day.
Add to the mix Covid, which has changed working patterns and the lack of full service airlines operating that size of aircraft with access to and the willingness to use slots on a MME-LHR and the likelihood is LHR a pipe dream. Its basic economics.
Remember how BMI manipulated the LHR when it was running in the 15-20,000 a month and managed to run the service into the ground by simply putting an Embraer on the route and messing with the timings, once they pulled off the route the 145s didnt then stay at LHR to use those slots vacated by the MME.