On 1 July, 12:31, Mizter T wrote:
None of the various Underground options are entirely step-free.

You can take the hourly Southern service from Watford Junction to West
Brompton. Walk across the platform to a District Line train one stop
to Earl's Court, where there are lifts to the Piccadilly Line, which
goes to Heathrow. Allow 2 hours all in.

Because West Brompton doesn't have a full set of lifts, to do the
reverse journey you need to catch a District Line train from Earl's
Court to Kensington Olympia instead (beware these are a bit less
frequent than other tube trains - worst case a 20 minute wait), where
the District Line platform is adjacent to the northbound mainline
platform (and therefore there are no steps), for the Southern service
to Watford Junction.

U