>I would never contemplate N or W London over SW or Surrey
Quite - Essex is a flat as...err.. a flat thing.
>Swinley I don't get when for about the same time/investment
Swinley trails are maintained by BOBMBC (Berks On Bikes MTB Club), plus they drain pretty well, so are rideable all year - certainly some of the better known N Downs sngletrack (ie not bridleway) I'd actually feel guilty about riding in winter 'cos they get such a hammering (tho' I believe there's some maintenance done on 'em nowadays). Not to mention there are cheeky (ish) trails around, too.
Chilterns/Henley way is (AFAIA) bridleway stuff (tho' there's an MTB club out that way too, so they'd know better).
Both Swinley and N downs you can get to on trains through Clapham Junction, Chilterns I think is Paddington (tho' I used to do mtb orienteering events around Reading/Henley/Sonning some years back, that would probably have been via ClaphamJ.)
Bear in mind that wherever you are within the M25 (or at least, the South West Trains bit) - the buggers won't, in theory, let you put a bike on the train around peak times (that includes about 7pm). However, if I'm riding at Swinley (I work out west M25, nr Staines) the train has emptied a bit so it's never a problem, despite the 'cycles are not allowed on this service' rubbish on the indicator board. But try putting a bike on the train futher in and you *may* get grief.
Re '10 mile, cheap, no crime'...pick any two
Actually, it's not that bad...
I'd go for somewhere that is at least on the mainline to Clapham - have a look at the tfl/nationalrail websites.
FWIW, I lived in Surbiton for ~15 yrs - 20 mins to Waterloo by train, you can even ride out to the N downs from there - but that's more like 16 miles out from the City IIRC.