Unfortunately this is definitely a ‘site of’ and absolutely nothing can be seen on the ground except for the east end of the North Runway at Heathrow Airport.
From the limited amount of evidence I’ve gathered so far, TQ091766 should be the area where ‘Caesar’s Camp’ was, as surveyed by William Stukeley in 1723. The Heathrow Visitors’ Centre says that the site was just beyond the east end of the North Runway.