As it says… thinking of riding in from Reading, which means I’d hit West London somewhere Heathrowish I guess

can you get on to the grand union canal?

http://jonchoo.blogspot.co.uk/2008/06/grand-union-canal-bike-ride.html

well you can more or less ride the a4 the entire way, but its not too pleasant

id head for the grand union slough arm you can follow that all the way to hayes (can stop off at the bmx track if u like) then take the paddington arm as far as paddington stn then head south through hyde park toward victoria

the paddington arm is a bit of a detour- the a4 from brentford is the straightest route – or possibly follow the thames, but its a bit bendy

IIRC there’s a sustrans route. Should be on their website

Looking at streetmap, A4 through Brentford seems to have a decent cycle lane all the way along it, for ages