As a West Londoner and PPL the following is a suggestion only ( will probably be shot down ... but hey ho):
When LHR is on 27R/L arrivals
Not below 2000-2500' (ie passing over the top of inbounds & helis)
Utilising all open spaces to comply with rule 5 as best as possible although even then your options are limited
Route 1
Stanmore Station - Northwick Park - Iron Bridge (A4020/A4127 + GWR) - Hounslow Heath - Sunbury Lock
Route 2
Denham - Northolt Apt (Overhead) - Iron Bridge (A4020/A4127 +GWR) - Richmond - Caesars Camp