The baseline for XC improvements really ought to be for every service to be a double Voyager; internal refurbs would be an added bonus!
I think you’d need all the class 220, 221, and 222 units to double up every service. IIRC there were 53 diagrams pre Covid, less diagrams now.
If you took on all 221s and 222s, and reformed the 222s a bit, you’d have:
34 4-car 220s
4 4-car 221s
40 5-car 221s
4 4-car 222s
17 5-car 222s
4 8-car 222s
Which they could aim to run as:
Around 35 diagrams of 9-car 220/221
Around 2 diagrams of 10-car 221
Around 4 diagrams of 9-car 222
Around 5 diagrams of 10-car 222
Around 3 diagrams of 8-car 222 (single units)
So around 49 diagrams of full doubled-up units total. Give or take a bit with the 9 vs 10 car combinations based on availability.
Gives 4 spare 220/221s and 4 spare 222s (one of which is 8-car).
Obviously this is pie in the sky and the DfT will never approve the funding.