At the very least it's hopeful that the RR V8 wouldn't need over a decade of 'get well' programmes costing who knows how much.
That said Israel was interested in the Chieftain before riots at British embassies in the Arab world gave the Brits cold feet. Iran bought almost 800, fostered the development of what became the Khalid and the Challenger, in fact I'd cut out the Anglo-German MBT and the MBT 80 projects and go straight to the Challenger.
It would at least not face the thermal issues of two-stroke engines nor the narrow rpm range needed for opposed-piston two-strokes (the L60 would have worked better if combined with a hydromechanical transmission which allows the engine to operate more often in the optimal rpm range for scavenging).
I think in hindsight Leyland would have been better off committing to either the V8 or V12 diesel. They could then either improve the power output of the V8, or reduce the complete powertrain size of the V12. Leyland certainly had the time to do that instead of starting from scratch with the L60. For all we know, the extra hull size required for the V12 may have been what was needed historically for the L60 anyway. At least either Vee would have been suitable for commercial use or improved developments in much the same way as contemporary diesels for use in future designs.
What sort of virtuous circle would be needed to get the MBT 80 into service and working properly before the British government started to worry that it would be too late?
My reasoning is that given the British got the Challenger the most efficient path would be to not waste effort on the Anglo-German FMBT and MBT 80 and just focus on the Challenger.
The MBT-80 program was plagued with management issues which delayed its introduction date from 1985 to 1994. It didn't help that nobody could agree on the components and materials it would use, and that the various services (RARDE, FVRDE, ROF) were not working well together. There was excessive involvement from the Minister of Defense, and it appears that the requirement were based more on what the study thought was technically possible rather than what the Army wanted (which assumes that the latter had less ambitious reqs).
On top of that the personnel was split between work on FV 4030 Shir 2 and MBT-80, which even caused issues with test rigs which tested features from both tanks.
Even then, MBT-80 as is or perhaps even some of the successor programs (MBT 95 and Challenger MLI) would have been better than Challenger 2 itself so that could have been worthwhile even if Challenger had been adopted, but I feel we have to go back well before that.
I do not have such a positive view of the Iranian orders. The Shir program was putting all the eggs of the British tank production capacity (and expanded at that) in a single basket until at least 1985, neglecting the other interested parties (India, Egypt). The Iranian revolution might have been unexpected, but this move was still very risky.
But IMO, the time we need to go back to to obtain a genuine virtuous cycle was 1969-71. The British wasted a very long time reflecting on how much they should technically stray from Chieftain, incrementing from FV 4211 to 4211 Mk 2 to 4222 (Shir 2 with aluminium hull and 110mm gun) to FMBTs (the latest inching close to MBT-80 in design).
If the budget structure over the 1970s allows that, they should have gone all in a future domestic tank program starting in 69-71 with no direct foreign involvement (no Germany, no Iran) and with a genuinely new design (of a complexity to decide on). This could notably leverage MVEE's 1969 study on engines (which recommended either a RR or Leyland V12 turbodiesel with commercial roots, so probably the origin of CV12), or the Leyland V8/12 of the Chieftain if this had been chosen, work on the 110mm gun (to be oriented towards a 120mm rifled gun with better ergonomics and construction than the L11, and with no need to use the same cartridge which crippled the historical L30. It's probably too early for thd UK to read the room about smoothbores), and Burlington armor.
This is the best way they have to achieve a somewhat competitive design available in time for the export cycle in the 80s, which doesn't need a supplementary program like CR2, and to have somewhat mature components to use in multinational negociations in the 70s.