If you want to avoid the central London shuffle or the tube, then you have a few options. These are the routes that can be used with a fully flexible Off-Peak or Anytime Return, but do note that advances can be sometimes found on these routes for less than half that cost, so it may be worth checking your options on Trainsplit before committing to a full-cost open return.
- Southern to Brighton, Thameslink to Farringdon, Elizabeth line to Stratford or Liverpool Street, Greater Anglia to Norwich (around 5h 30)
- Southern to Chichester, Southern to Three Bridges, Thameslink to Farringdon, Elizabeth Line to Stratford or Liverpool Street, Greater Anglia to Norwich (around 5h)
- SWR to Woking or Winchester, SWR to Clapham Junction, Mildmay line to Stratford, Greater Anglia to Norwich (around 5h)
For comparison, the most direct option (SWR to Waterloo, Jubilee line to Stratford, GA to Norwich) takes around 4.5 hours, so avoiding central London will add 30-60 minutes to the overall trip time.
If London Crosslink was still around (direct Basingstoke-Essex train services until 2002) then that would be the most straightforward option, but it still wouldn't have been particularly fast.
Is it possible to change at Clapham Junction?
If you prefer sitting on the Overground for 45 minutes over taking the tube, then you can do that with a flexible ticket. However, as the direct trains from Cosham to London skip Clapham Junction, you'd have to change trains an additional time at Winchester or Woking to get there.