DB Cargo are hoping they will be winning the contract along with the associated extra Daventry - Scotland stuff.
That said, nobody could say with certainty whether the existing London - Warrington and London - Warrington - Scotland trains would just stop at Daventry on the way, or if they would require new services in their own right.
As it stands, you only have a daily through 12 car between Willesden and Scotland, with an extra Willesden to Scotland (and vice versa) that officially changes headcodes and service at Warrington each way. That, and the daily Low Fell. If Low Fell is still a 4 car, that’s 2 units used on that. They may still also do an in-fill Warrington to Shieldmuir extra if parcel volumes require.
DB predicated their whole thing on a few 321s being added, and converted, to be able to provide the extra capacity needed. I don’t think Royal Mail had planned to buy these. But if Daventry is going to be extra, you will definitely need more units. It’s just counting, not least when you add in the needs to maintain and move stuff onto/off the Low Fell diagrams.