That's what I considered with my idea of Liverpool to Brighton via WLL (Watford Junction - Kensington Olympia - Clapham Junction - East Croydon) with Clapham Junction being a crucial interchange from this otherwise through service. Since the BML has barely any spare capacity, it might be easier to go to Portsmouth Harbour instead from Clapham Junction. Actually, that's not a bad idea given the low variety of services from Liverpool these days compared to Manchester and Leeds, as well as the fact Portsmouth lost XC services years ago (tbf so did Brighton before that). Liverpool Lime Street to Portsmouth Harbour, which involves London in the form of Clapham Junction, calling at: Runcorn, Crewe, Stoke-on-Trent (add another connection opportunity), Lichfield Trent Valley (or Tamworth / Nuneaton), Milton Keynes Central, Watford Junction, Shepherds Bush (for Westfield London), Clapham Junction, Woking and finally Portsmouth (either terminating at Southsea or continuing to Harbour).