I'm also curious now as to whether Hillingdon area folks ever try and
use the Oxford Tube/Espress as a nifty 'express' night bus to get from
central London back to Hillingdon.

Perhaps 'local' passengers are only really welcome to travel from
Hillingdon into central London, rather than the other way round.

This reminds me of someone telling me about how they used to travel
into town from their home in the London suburbs back in the 70's - the
normal way was on a regular London bus route, but if they were feeling
flush then they might opt for the quicker Green Line coach instead
(regarded as a somewhat cut-above way of doing things!). I can't
really think of anywhere where this might still be possible - bus
routes are a lot shorter these days of course, and long distance
coaches don't really stop at that many places (though the Bexleyheath
coach stop on the side of the A2 does come to mind).

I used to live in Hillingdon and yes it was possible to use the Oxford
Tube/X90 as a night bus from Central London. It was around £4 I
believe (about two years ago) and it was absolutely brilliant. 20
minutes from Marble Arch compared to 90 mins or so for the N207, which
was further away from my house. Well worth the extra few quid.

I don't know if many other people used it, though. The times I used it
there never was anyone else getting off at Hillingdon.