If the Great Central had not closed, would it have had HSTs like the Midland Main Line?
Even the MML only got second hand HSTs when InterCity prized some away from the WR, who by 1983 were running them on secondary and even local services.
The GC, by then a 75mph secondary route, would not have figured high on the priority list, and would probably have gone down the Chiltern route with DMUs, or possibly LHCS like XC, in the 1980s.
It might have become a 158-serviced route perhaps, maybe feeling like today's LSWR Waterloo-Exeter route.
How far it went beyond Sheffield would have depended on the fate of Woodhead, which lost out to the Hope valley route as the main route to Manchester.
The MML has had significant upgrades from its 90mph since HSTs were introduced, making 125mph stock worthwhile.
It's hard to see the GC line receiving anything like those upgrades.
Edit: In fact, BR pursued a singling policy on downgraded main lines, like Salisbury-Exeter, Oxford-Worcester-Hereford and (close by) Princes Risborough-Aynho Jn.
So I'd expect the GC would have been singled north of Aylesbury, or perhaps Calvert.
That in itself would have extended journey times, especially if token exchange was used as it was on Moreton-in-Marsh-Evesham-Norton Jn for 40 years.
Then there's the uses to which the GC trackbed has been put over the years.
I'm not familiar with the area but new roads (M1 was it?) and things like the Nottingham trams would not have been able to reuse the GC route.
No shopping centre at Nottingham Victoria either.
The Yorks/Notts colliery closures in the 1980s would have impacted freight services on the line.