Despite having a layman's understanding of the class 373, surely it would be relatively cheap to re-engineer them into HST replacements compared with the price of a new fleet. I can't imagine it would require much more than shortening by a few cars, and perhaps replacing the motors combined with a new/updated firmware. If it is seen as worthwile to re-engineer old London Underground stock, old class 47s, 37s, and 73s to fulfill new purposes, then I cannot see why 25 year old modern high speed units are not seen as worthwhile.
Of course, with new class 800/801s they are no longer required even if they could have been a cheap HST replacement

However, some could potentially go to Anglia, as they are far more modern than 225s (are they still the planned replacement for the class 90s?).