In the Upminster example provided, I would consider a small taxi journey
reasonable enough; Upminster to Barking or East Ham say, but not Ealing
Common or Neasden. But common sense needs to work both ways and
LUL management shouldn't be expecting drivers to book-on at such distant
places in the first place.

I wonder if this can be solved by defining a group of primary depot
and several secondary depots/stations nearby for every train operator.
Like: Upminster/Barking/East Ham, Edgware Road/Earls Court/High Street
Kensington, etc.