As an aside, I think the Northampton train you referred to was withdrawn some time back, possibly pre Avanti? Certainly there are no Avanti services showing on RTT that I could find.
It was running right up to the first lockdown, cancelled then reinstated for a bit (and withdrawn again)
It's origins date back to the LH Northampton - Euston services, where latterly an Oxley - Euston (IC) ECS was hired in by NSE from Northampton to provide one of them. The current arrangement is use of a Branston - Euston ECS service; originally into passenger service at Northampton, but moved back in stages to New St, (for those who prefer the scenic route - it's 'overtaken by a later train').
The Wellingborough café had a sticky period when NE MML offered free tea and coffee in both classes (not continued by Stagecoach) and, presumably, they welcome the further changes. Their cups may be larger, as well.
The Milton Keynes - Euston first class commuters do seem very keen on their at-seat morning coffee. Personally, I allow enough time between getting up and leaving home to make my own...