There seems to be a rectangular cropmark around the east end of the northern runway at Heathrow, which could be Caesar's Camp, pictured in the aerial photo of Heathrow in the BBC book "Britain from Above". I cannot post the image as it is BBC copyright.
‘The Archaeology of Hounslow’ by Gillian Clegg (1991 – ISBN 0951831901) says “William Stukeley…..made a drawing of one of the few prehistoric field monuments in West London – Caesar’s Camp at Heathrow….Since this site was damaged by ploughing in the 19th Century, Stukeley’s drawing was valuable evidence for the archaeologists who dug the site in 1944 prior to the construction of Heathrow’s Number One runway.”
“One of the most interesting settlement (sic) at Heathrow lies directly under Runway One. Here, excavations in 1944 unearthed an Iron Age village containing 11 hut circles and a rectangular building, thought by its excavator to be the remains of a shrine or a temple. Since Iron Age temples are uncommon in Britain this find has attracted a great deal of interest. It suggests perhaps that the site functioned as a religious centre for a larger community than actually lived within the settlement. The earliest phase of huts dates to the Bronze Age. The huts were subsequently re-sited and surrounded by a massive bank and ditch which was still visible in the 19th Century and from which the site derived its nickname of Caesar’s Camp.”
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.