Oyster 60+ and out of boundary 6

I am intending to travel to Shoreham Kent on Thameslink and I know the oyster 60+ finishes at swanley. I will buy a return ticket from swanley to Shoreham.

What happens on the return back to central London. Do I need to tap in at swanley for the oyster to be active or stay on train until my desired stop

If staying on is the answer. What happens if revenue check the ticket as it will not have been activated for the return as well not showing an exit at swanley on outward journey.