Lew 1 wrote:

Not necessarily. From St Albans I can get a ticket either to
Farringdon Und, route Not Underground, or to London Thameslink. ISTR
that the latter did not work the barrier at Farrongdon but was
accepted by the barrier staff.

Out of interest, which one is cheaper?

Provided that the London Thameslink ticket is also route Not
Underground, they are sold at the same price. The London Thameslink
ticket even allows you to go as far south as London Bridge/Elephant &
Castle.