The fighter aircraft are there to tackle the issue long before anything gets to the urban sprawl. Dont forget things happen very quickly at altitude. It's not the first time I've looked at London from the south coast as I'm running the checklist.
Regards whats supersonic at the treetops. Aardvark will do 1.2mach, not 2, though it's not, and never has been, a fighter. It simply got the 'F' desig as it was born from the debacle that was TFX and it carried the funding over. It has never operationally been a fighter, always a mud mover. The cancelled B desig was capable of carrying the Phoenix missile, but that aircraft was cancelled. We all know where that missile and the radar that guided it went though, so that worked out ok in the end.
If we're bending the rules a little, the F3 Tornado (pre restrictions) would shuffle away from the 'vark at treetop height on full chat. Mate who was a backseater on one tells substantiated claims of serious speeds when clean. There is a small, small club out there (that he is not a member of) that have claim on four digit IAS at oil rig height over the sea.
However, considering the 111 first got air under it's bum in '64/'65? and the F3 is a child of the 80's, kudos to the team at NASA that sorted out the inlets on the 111 and made it the aircraft it became.