Alternate?
One curiosity for me that hasn't been discussed is that 20 mins into the flight would mean they were well enroute (assuming the OP meant 20mins from t/o) and with the compromise to safety being speculated why they didn't go to East Midlands or Manchester - would that not have been a quicker way to get on the ground than returning to LHR? This suggests to me that whilst the crew were concerned it wasn't a critical/'Mayday' incident....