Mizter T wrote:

I presume terminating the other off-peak trains at City Thameslink and
then perhaps parking them up in the Smithfield sidings until they're
due to head south again is considered impractical given the frequency
of trains on the core Thameslink route.

That is true, and according to Nick Lawford the sidings will close when
the Key Output 0 service starts.

At Kentish Town I presume the benefit is that the fast Bedford trains
will be on the fast tracks, hence there's less opportunity for
terminating/reversing trains to jam up the whole Thameslink service.

*All* cross-London FCC TL services use the Moorgate (TL) lines south of
Kentish Town Jn (between West Hampstead Thameslink and Kentish Town).
Using the same point of reference,* services on the fast and
slow/carriage (MML) lines reverse at London St Pancras (high-level) and
Kentish Town respectively.

It is expected that off-peak joint FCC TL/SER services will
start/terminate at Kentish Town on the Moorgate lines, but only by
virtue of heading ECS to/from Cricklewood sidings.

[* OK, Carlton Road Jn (also between WHP and KTN) is the point of
reference for the fast lines since Kentish Town Jn does not include them]