I reckon I can best all comers with this one.
St Budeaux Victoria Road to St Budeaux Ferry Road.
Distance (as the crow flies), the width of a small road.
By train; to/from Plymouth, or occasionally a connection at Devonport. This would not qualify, except that on Mondays to Fridays there is a direct train, departing Victoria Road at 1803 and arriving Ferry Road at 1825. The fare is £13.00!!!
That has to be the most expensive / mile journey it is possible to make on the UK rail network on one train.
There will be some similar ones on the loop services around suburban London which are pretty daft. However, neither your example nor these are the shortest possible journey purely when mileage taken by any given passenger service, or time, for that matter, is taken into account.
You don't need to go via the M23. There looks to be a route possible heading west through Horley to get to the A23, then head south to Gatwick. Still a very indirect route, and likely very slow.
Of course not, and in fact Horley has no junction on the M23 anyway - but the moment the M23 has a problem, it spills over onto the A23, which will in turn impact most feasible routes from Horley to Gatwick.
Even if the trains are pretty heavily disrupted, Horley-Gatwick is still usually quicker on the railway - or on foot, in fact! Quite a few people do it as a commute by train.
