We have a thread on Tonbridge-Ashford, but the broader context of that route (certainly to Redhill, straight and fast) being underutilized.
But previous services to Ramsgate and so forth, from Cross Country - used to go to/from Kensington Olympia direct to Bromley South (about 25 mins) - didn't call at Clapham. And I remember many going directly to Watford Junction, and WCML, instead of via Reading. All manner of combinations, even some Newhaven and Dover.
Since these quirky, charming days - frequency on many routes, but notably orbital ones (WLL, ELL) which are needed for routes like this, are all 4tph+ and freight hasn't gone away... Catford Loop, Herne Hill, they're all much busier so I don't think it would be easy to plug these weird and wonderful services into our boring, yet far more reliable clockface world.