The track at the beginning of this thread shows a easterly departure from LHR 9R and assuming a standard Compton departure the captains estimate of about 8 miles from Heathrow puts the plane just south of Lego land themepark situated in the middle of the crown estate at Windsor. That would fit perfectly with estimates that the l@ser was 1-2 miles off to one side- ie to the north of track.
Sorry if i missed it but do we have now a firm confirmation that the aircraft was around 8000ft ? If so it must have a very powerful beam, something way beyond what could be laying around Legoland...