I think from now on I might just make a point of doing that all the time! It'd be quicker than waiting for them to select a Gatwick Express ticket so that I can press "start again" and buy the ticket I want once they've gone to mug the next customer!
For an off peak return journey an East Croydon Travelcard and Gatwick to BZ6 return is cheaper than the ticket the helper suggests and is valid on Gatwick Express; both can be bought from the TVM too.