That's FCC, it could get busy but that's quite early so you may find a seat. Someone who commutes from South London into the City may be able to answer that (I'm sure there'll be someone here who does!)
Assuming it's an 8 car train, if you go to the very front I'd be surprised if you don't get a seat.
Board at the back with the masses and it could be a squeeze.
I'm not so sure, there. Having looked at the timetable, this train - the 0607 from Brighton - calls at
every FCC station, even Redhill which only has a very limited FCC service. Whilst it is true that most would board at East Croydon towards the rear, and the same could be said at Brighton to a degree, there are many other stations en route. The service is booked to run as at 8-car train, so you should be able to find a seat, but don't expect to every time. Also, from St Pancras (remember you can change before there if you choose to, but not before Blackfriars), the next service to Mill Hill Broadway is an 18 minute wait. It runs from Sevenoaks to Luton.
Interestingly, although the 0709 from East Croydon calls everywhere
south of London, to the north it does not. Beyond St Pancras, it would appear to be a "Luton Express", i.e. Luton Airport Parkway and Luton only, skipping even St Albans.
