A few thoughts:
1. The problems on the Harrogate line (putting 100mph Turbostars on a slow route where they struggle to get out of second gear) clearly hasn't taught people anything (given the ideas about putting badly accelerating 125mph trains on slow stopping routes with no fast sections)
2. The S&C is roughly a five hour round trip - for a bi-hourly service that means just three trains required. It's a rounding error, in the grand scheme of things, when people come up with solutions that involve putting spare 185s on it (though I agree that a 185 would be a lot better on the 75mph S&C than trying to get a 125mph HST to cope with at least fifteen intermediate stops).
3. If your idea to find a use for redundant stock involves creating new services then it's probably not going to happen
4. If your idea to find a use for redundant stock involves building (or re-building) lines to accommodate those new services then it's definitely not going to happen
5. I like the idea of a faster/better Newcastle - Carlisle service, but the line is getting their 142/156s replaced with "Connect" 158s and a second service per hour over the Pennines, so upgrades are happening anyway
6. My understanding was that the monocoque MK3s wouldn't be suited to fit wide doors into, hence being unsuitable for "Parcel" work. Whereas a 319/769 has nice wide doors at 1/3rd and 2/3rds so much more suitable for lugging large volumes of parcels into/out of.
7. Ten out of ten for imagination, mind