Use the "more search options" feature on the journey planner and specify
that you require no stairs under the mobility requirements. An
alternative shown is getting off at London Bridge, taking the Jubilee line
to Waterloo, then taking a mainline train direct to Richmond. That
involves escalators rather than stairs.


At London Bridge, stay on the same platform and get the next train to
Waterloo East.
Waterloo East has a ramp up from the platforms, a level footbridge, and then
an
escalator or lift down to the main concourse. No stairs at all.

--
Peter.

Peter Goodland wrote:

At London Bridge, stay on the same platform and get the next train to
Waterloo East.

That applies ONLY if you take a First Capital Connect (formerly Thameslink)
service. If you take a Southern service from Gatwick you will end up in the
bay platforms - however, it's still a level transfer via the barrier line
and the subway (which is ramped) to platform 6, where the Waterloo
East/Charing Cross services leave from.