Motorway cantilever sign
A blue overhead sign suspended over the motorway from a post on the left, with a downward-pointing arrow indicating the left-hand lane, and the following text.
A312 ; Heathrow [Airport symbol]; Harrow; Hounslow.
Junction number: 3.
The A312 is what's left of a planned ring road. It won't let you orbit London, but it will get you to the A40 reasonably quickly if you need to switch to a different approach to London.