Jon wrote:

A few years ago there was a circulat bus route linking all the main
London stations, using low-floor vehicles with loading ramps. It was
indeed slow. The idea was to serve travellers who needed ro-ro loading
for wheelchairs, prams, etc and so could not use the Underground at
all, although the service was open to all users. I think (someone here
may know better) that it disappeared for lack of customers. Perhaps few
wheelchair users make cross-London journeys or maybe most doing so
prefered a taxi transfer.

That's correct: in the 1990s there was a pair of circular services,
Stationlink SL1/2 (clockwise/anti), although there had been something
prior to that as well which failed (and ISTR a night-time service in
the early 1980s). These were pretty infrequent and because they served
virtually all stations could be terribly slow - it might take a couple
of hours from arriving at one terminal before you reached your
cross-London terminal. I think they disappeared around 4-5 years ago.

Route 205 is a legacy of the Stationlink service, and there was also a
705 which seems to have withdrawn. Apart from the slow journey times,
taxi options and infrequency, the accessibility of all London buses to
the groups mentioned above probably put the final nail in the coffin of
dedicated Stationlink services or similar.