Andrew writes
Then don't sell the ****ing tickets if they can't deliver.

Once again proving your ignorance.

Travelcards are valid on that route and Travelcards are sold by lots of
train companies and TfL. Thameslink can't prevent those people with
Travelcards from travelling on its trains even if it wanted to.

Exactly. The same perhaps applies to a fair number of single and
return journeys within Thameslink's operating area, eg a single Kentish
Town to London Bridge might be sold at Kentish Town LU station, but be
valid on Thameslink services too.

In fact, to add to the above - I have a feeling that for the section
between Kentish Town and London Bridge/Elephant & Castle/Moorgate, LU
fares might apply anyway. I've always assumed that's the reason why
railcard discounts don't apply for journeys wholly within that section.

--
Dave