MIG wrote

Today, while queuing at the ticket office (with a slightly shorter
queue than the machine with a pad) purely to find out how much credit
I had left, because the barriers don't display anything, a couple in
front of me were being charged £8 to get to Finsbury Park and were
shocked.

They lived in Cambridge and had no intention of getting Oyster cards
(£6 in deposits?). Welcome to London.

So why didn't they buy a through ticket to Finsbury Park at Cambridge ?

--
Mike D