I searched trainsplit.com for Tonbridge to London/Stratford with a change at London Bridge on the outward and at Ebbsfleet on the return and it gave various options, all using single tickets, so I then split the search into two parts and looked a bit closer.
Out: Tonbridge to London Terminals Anytime Day Single NOT HS1 £18.40, or save £1.80 by splitting at Chelsfield or 20p by splitting at Orpington, if you take a train that stops there. This gets you to London Bridge, and then you take the Jubilee Line to Stratford (regional) and walk/DLR to Stratford International on Oyster/contactless (£3.00 if tapping in before 09.30, and £2.50 otherwise). This can be marginally cheaper than the Tonbridge to London Undergrnd Zone 1-2 Anytime Day Single
£21.40 which includes the leg to Stratford.
Return: London Terminals to Beltring Anytime Day Single Plus High Speed £25.20 + Beltring to Tonbridge Anytime Day Single £5.10 for a total of
£30.30 is valid for Stratford International (starting short) to Ebbsfleet (break of journey) to Strood to Beltring (make sure train stops there) to Tonbridge. (You could use the otherwise-unused part of the ticket on St Pancras to Stratford on HS1 in the morning, although it's probably slower than taking the Jubilee Line.)
If you want to save money on the return journey, instead of HS1 from Stratford to Ebbsfleet (10min) it's possible to take DLR to Woolwich Arsenal (25min) followed by a train to Northfleet (30min) which is a 15min walk from Ebbsfleet. The DLR leg costs £1.60 on oyster/contactless if you touch in before 16.00, then you can get a London Terminals to Paddock Wood Super Off-Peak Day Single NOT HS1 £16.50 starting short at Woolwich Arsenal and breaking at Northfleet, followed by Paddock Wood to Tonbridge Anytime Day Single £4.80, for a total of
£22.90.
There are tickets available that include HS1. I'm pretty sure you can travel either way with these tickets so that should be the solution.
Bizarrely, I'm unable to get a journey planner to confirm that Tonbridge to London Terminals (or to Stratford International) Anytime Day Return Plus High Speed £75.00 (or £74.90) is valid on HS1 (searching Tonbridge to St Pancras via Stratford International or to Stratford International via Ebbsfleet International just yields no valid results or an offer of a combination of tickets), but I can't see how it couldn't be...
(there seems to be no valid route via Maidstone)
I got trainscanbecheaper.info (which allows multiple via points) to validate Paddock Wood to St Pancras or Stratford International Plus High Speed via Ebbsfleet and Maidstone.
Question to the forum experts: Is there any way of getting a return ticket that is not valid on HS1 for the outbound but valid on HS1 for the return, at a lower price than a full HS1 return ticket (either from Tonbridge or Paddock Wood)? I'm not too familiar with change of route excesses and High Speed supplements...