Thanks for the reply.

What about touching a reader at Farringdon?

In this video https://youtu.be/vfUzLVTDuig?t=461 the person is going from Gatwick to Heathrow using Thameslink and Elizabeth line but at 7:41 they say that you need to touch the Oyster reader to pay the right fare, why is that?

What would happen if this person touched in at Gatwick and touched out at Heathrow without doing anything at Farringdon?