Presumably something attracts people to declassified first on a 700. Assuming it’s not an attraction to antimacassars, it can only be either the presence of tables or the plug sockets.
I find it's mostly because of the plug sockets when I travel on them, mind you there must be a third reason that hasn't mentioned...
For me, a combinaction of the better seat (as I tend to do longer journeys on them) plus the plug socket.
I'm the same, recently I've had a 2.5hr commute one way on them so I generally make my way to the declassified 1st Class and make myself comfortable, the slightly better seating and the plug sockets are my main reasons for using it.
Table and bigger seat in my experience. Very rare to see the plug sockets in use, indeed I don’t even know where they are!
Plug sockets are under the seats in much the same way that they are fitted on Class 717s and also how they're fitted on the SouthEastern HS1 Class 395s. Usually a pair of plug sockets per two seats.
So which longer-distance services were planned to be an 8-car service group? I still can’t think of any.
Whilst Rainham to Luton may be longer distance, I’d still put this as a metro route.
Even though it's a 2.5hrs end to end, it is a metro route as it stops nearly everywhere from London Bridge to Rainham but is semi fast north of St Pancras.
All the infrastructure on Rainham-Luton is fit for 12-car operation (with SDO at some short platforms), so (IIRC) would likely be 12-car if there were sufficient stock.
No it isn't, not all the infrastructure is fit for 12 car operation especially in Kent.
As an aside, whilst Rainham to Luton is clearly 12-car capable, I suspect there might not be sufficient stabling for that whole service to go 12-car if units were to be lengthened? The stabling situation on the Midland side is already messy with 8-cars having to find their way up to Bedford and 12-cars down to Cricklewood.
I hate to burst your bubble guys but Luton to Rainham isn't 12 car capable at all because some stations are only permitted to take a maximum length of 10 car trains even if the actual platform length is longer then 240m.
So regardless of the Class 700s having SDO or not, it's not permitted by Network Rail so not worth discussing 12 car operation on the Luton to Rainham services.
Higham 10 car only
Woolwich Arsenal 10 car only
And that was taken from the Network Rail (Kent, Wessex, Sussex) Sectional Appendix dated Feb 2021.