I am not sure if the APT programme would have made much difference to the HST. We would probably have not seen the 91s and mk4s. But the HSTs we already committed before the APT project was finally killed off. In fact the ecml electrification was not approved until after the APT was killed.
The Intercity 225s were directly developed from the APT project. Even as early as 1973, the APT development team had created a proposal for a smaller, 125mph APT-S with a single power car, but at the time they were busy concentrating on the development of the twelve carriage, 155mph monsters that were the P-trains. By 1982 when some experience had been gained through the testing and operation of the APT-Ps technology had moved on and the power car for the proposed APT-S had been moved to one end of the formation: Essentially giving the Intercity 225 formation of loco, nine carriages and a DVT. At the same time the design for the "full fat" APT-U, featured power cars at both ends of the train sandwiching ten trailers for 140mph operation, and the idea of articulating the carriages had been dropped.
It is therefore clear to see that production APT sets would have had much more in common with the Intercity 225s than the APT-P sets. The project was not simply "killed off" and attention switched to developing the Intercity 225: Development from one to the other was continuous: When cessation of the APT project took place in 1985, one APT-P trainset was officially redesignated as "APT-D" for continued development work in connection with the creation of the class 91s.
It was quite early in the project that it was admitted that 155mph running offered very few benefits over running at 140mph, offering only a 3 minute time saving on a Euston to Glasgow run. And later on, by 1982, it was recognised that even 140mph running would only be possible if the old chestnut of "improvements in signalling" was achieved, and the production APT sets would probably be limited to 125mph to begin with, which sounds familiar on both the East and West Coast main lines at present...
If production APTs had been introduced on the WCML, then I could envisage that the class 90s would not have been introduced. Only "roarer" classes 81 and 85 really survived to the stage where they were replaced by new traction developments later than the 87s, with the 85s hanging on the longest until replaced on Speedlink duties by 90s. If APTs had been introduced on the West Coast in the mid to late eighties, then 36 class 87s, only 15 years old, would have been surplus to requirements, which could have handled the freight work that was taken on by the 90s. West Coast only really received a number of 90s for passenger work as recompense for the lack of investment in new rolling stock as a result of the cessation of the APT project.
After electrification through to Glasgow in '73, the West Coast rather seemed to miss the boat when it came to rolling stock modernisation: First with the cancellation of the APT project, then by missing out on a follow on order of tilting Intercity 225s (Tilt was never considered for the East Coast 225s, as it would only save 5 minutes on the London to Edinburgh journey time, but the trains were of course originally developed, as a follow up to the APT project, with the West Coast in mind), and then with the proposed Intercity 250 which re-introduced the idea of 155mph running that had been mooted 20 years before.
It is interesting to consider whether Virgin would have replaced a fleet of APTs that would have only been about 20ish years old: Surely one of the driving factors behind the replacement of both the Crosscountry and West Coast fleets was that there was such a hotch potch of different train types operated by both franchises (47s, 86s, HSTs and mark 2 coaching stock on XC, 86s, 87s, 90s, HSTs, mark 2s and mark 3s on West Coast) of which the oldest members of the fleet, the 47s, 86s and mark 2s, were all in the 30-40 years old age bracket.
Though Virgin did have big plans for new trains whenever they have bid for the East Coast franchise, which operates the even younger 225s!