John Rowland writes
You can get to Slough (route 81), Dorking (route 465),
Redhill (route 405) and Watford Junction (routes 142/258)
by Travelcard - but only on the bus...

That's fantastic - all the way to Slough (and back) for £2
from south London, in about 2 hours.

GBP2 buys a Bus Pass, not a Travelcard.

Makes no odds, One Day Bus Passes also valid on TfL routes outside
Greater London.

(In fact any 'all-zones' bus pass is valid on such services - as are
*all* Travelcards, irrespective of zonal validity.)

--
Dave