Ah, thanks folks. So taking the advice of @Simon Davey to go west, I could try something like this:
https://www.google.com/maps/dir/Unit+1A,+Osprey+Group,+Thornhill+Works,+Horsham+RH13+8QN,+United+Kingdom/Belfast,+UK/@53.4273118,-7.0512274,6.44z/data=!4m19!4m18!1m10!1m1!1s0x4875bf3117300077:0x471370fe84f676d0!2m2!1d-0.4139893!2d50.9969548!3m4!1m2!1d-3.4855553!2d55.0608475!3s0x487d350e081ac8d7:0x4276062e4d90dfa6!1m5!1m1!1s0x4860fffdd7d08a3b:0x2e57162cefc7c531!2m2!1d-5.93012!2d54.597285!3e0?entry=ttu
(I learned that my starting point from where the motocycle is stored is actually pretty far from Uxbridge and the A40)
Whereas if I went North first, seeing @bonio 's point about the map, could be like this:
https://maps.app.goo.gl/d5tJZhMt2GUWuf7o6