So why would they buy a unit that goes slower than the ones they have?
There is no option, at least for now.
Alstom will not offer a tilting option, not least because they do not want to re-write TASS, which is obsolete and unique to the UK.
Other tilt options theoretically exist, but Network Rail/ORR won't allow tilt without TASS or equivalent to control it.
Avanti knew what the situation was when they bid for the franchise with non-tilt stock, but funded the line speed revision for MU stock (which covers 80x).
HS2 trains face the same conundrum, and will have to deal with the Weaver Jn-Glasgow section which is not being signed for MU speeds, at least yet.
Roger Ford has a fascinating analytical piece on all this in March Modern Railways.
This includes a detailed comparison of the performance difference between 390 and 80x.
He also mischievously wonders how a Pendolino at 140mph would perform on HS2 (ie the inverse of HS2 trains running on the WCML).
The time loss on London-Handsacre at lower speed would be regained by using faster tilt times on to Glasgow.