Ideally there would be more disciplined deployment of the 8-car and 12-car trains. 8-cars just on the inners with First Class stripped out. Outers exclusively worked by 12-cars with First Class at both ends.
If LU can keep S8s and S7s on their intended routes, there's no inherent reason why TSGN can't do the same.
There’s three immediate issues with this
Firstly, there will always be some requirement for 700/0s to reach Three Bridges as that’s where the depot is. And I don’t think there are quite enough 700/1 for the entire service to be provided by them.
Then you have KX-Cambridge which is about 75% worked by 700/0s, along with currently the bulk of the Peterborough peak extras. These currently all advertise first.
Finally there’s the issue of what happens if a 700/1 is unavailable and a 700/0 covers, as happens fairly routinely.
None of this is insurmountable, but is any solution better than the current situation?
One of the errors made by the Thameslink Programme was ordering too many 700/0s. Some of these are now serving Rainham, but the remainder are stuck on GN-only duties where their crowd-hoovering capabilities simply aren’t needed. As an aside, an associated error was the train crew depot which has no trains (Ashford). Of course, the DFT knew what they were doing, just like they did with the 700 interior spec…