168: if you're lucky, a Chiltern from Marylebone to West Ruislip. Otherwise you'll need to go out as far as Gerrard's Cross or High Wycombe.
171: London Bridge - East/South Croydon
180: You'll need to go as far as Reading on a select few GW services (see
here
365: Lots of them running from King's Cross to Finsbury Park.
379: Liverpool Street - Tottenham Hale
442: A few southern services to Victoria - East Croydon (or v/v), or a BZ6 - Gatwick is pretty cheap and run every 15 minutes.
444: London Waterloo - Clapham Junction, but they're mostly pick up (or set down for v/v) only, so you'll have to jump on/off without anyone noticing!
90s and Mk 3: You could try and do London Liverpool Street- Stratford, which every other train does, but it's set down/pick up only (as above). Alternatively, there's 1tph that stops at Chelmsford, which is just outside.
Hope this helps!