Hi again...this:
https://ramblingman.org.uk/walks/lon...ss-to-uxbridge
sounds remarkably like what you are intending to do...the comment about a security guard rings a bell...pity there’s not a map...
If you decide to start closer in to London you might be able to pick up your planned walk north of the A4...as covered in the “Crawford and Avenue Park walk here”...
https://www.walkingforhealth.org.uk/...s_hounslow.pdf
Tube to LHR Central, take a bus towards Hounslow down the Bath Road/A4...and either get off at New Road ( if you want to stay in the free fair zone, or pay and get off a bit further east opposite the Premier Inn.) .walk east down New Road and you can access the southern end of Cranford/Avenue Park...that footpath is signed.