They do sell a Flex option on airport transfers, but they will allow you on on non Flex tickets, particularly between T5 and LGW N since no-one can get on in between, so one empty seat is all you would need. There is some hassle attached to deploying a Flex ticket so on that route at least I'm not sure it's worth paying extra, plus the drivers don't always check the details of the ticket anyway. The other option is to buy via the NEX app near to departure. It's a less than ideal process since the app is a bit fiddly, and there's something like a 10 minute cut-off when you can no longer buy a ticket for the next service. But it allows you to be flexible as to what to do after landng into the airport. If there is an accident on the M25 then you can use the Elizabeth line / Farringdon service instead, which is around 80 minutes in all and you won't have a wasted NEX ticket.