Worth pointing out that St Pancras isn't a valid London Terminal from maidstone Stns unless the ticket is routed "plus hs1"
It makes no difference in this case, but from a booking engine point of view, I'd argue it's the other way around: HS1 isn't a permitted route on a fare marked "Not valid on HS1" (which is unsurprising!)
There is no requirement for a routeing of "Plus HS1" for HS1 to be a permitted route. Now I know you know this, but some people may not realise that, so I felt the point needed to be made in case of confusion with other tickets.
If you only going in one direction via HS1 it may be cheaper to buy a "not hs1 ticket" and pay the hs supplement but there are then complications getting between City Thameslink and St Pancras Low level.
True; a ticket routed via HS1 is valid via Thameslink, but I am not actually sure if a Not HS1 ticket plus supplement would be (of course these can't be issued by booking engines so it never crops up electronically). Does anyone know?
That said I suspect PeterY will be using an Oyster card (Freedom pass?) to get to the relevant terminals anyway.