In those halcyon Concorde days, I was living just west of Cardiff airport, au bord de la mer. The jet's 1030 LHR departure would, in summer, cause a series of booms which were clearly audible in this area about 15mins BEFORE the jet was overhead, westbound.
Concorde would light the burners as it crossed the Welsh coast en-route to NY, passing M1 rapidly off-shore. The jet was often clearly visible overhead, day or night (burner flames).
The most plausible explanation I could find for this seasonal but often daily phenomenon was that shock waves from the fwd fuselage & fin root were being generated by the jet at high IAS, though the jet was obviously subsonic between LHR & the Welsh coast. These sound waves were then preceding the aircraft.
Thoughts, anyone?
HB