You could buy a return from Plumstead to Dorking Stations. For the return journey you'd need a single from Caterham to Purley and would 'start late' on your Dorking ticket there. This would cost a total of £18.30 + £3.30 = £21.60 before Railcard discounts, assuming travel is after 09:30. If travelling at times when the Evening Out or Sunday Out fares are valid, that reduces to £17.50.
Alternatively, if you're happy to stick to Southern services to Dorking, you could buy a
Dorking to London Zones 1-6 Off-Peak Day Travelcard, route "Southern only", costing £18.10 before Railcard discounts. You would be 'throwing away' the outward validity from Dorking to Boundary Zone 6, but the ticket remains valid for further travel in Zones 1-6 even after you reach Dorking - so you should make sure you ask to keep the ticket if the barriers are in operation at Dorking.
If cost is of the essence, you could consider using Oyster/contactless PAYG as far as Epsom on the way out, touching out and then re-entering the barriers with a single to Dorking. It's admittedly unlikely you could make it back onto the same train given the relatively short stop at Dorking and the time taken to touch out and re-enter the barriers. On the way back from Caterham you could use PAYG all the way.
That would cost a total of £6.10 + £5.80 + £5.00 = £16.90 before Railcard discounts, assuming you touched in at times when off-peak fares apply, and travel on National Rail services via London Bridge. Cheaper fares are available, saving £3.20 in total, if you avoid Zone 1 (e.g. by changing at Surrey Quays or walking between New Cross and New Cross Gate).
