First, it's a well written and coherent TL so many thanks
@powerab
I struggle with a 1968 coup - a 1975 coup might have been more realistic in the aftermath of the 3-Day Week and the failure of the Heath Government to enact the radical proposals in the Selsdon House Manifesto.
I suppose there's a European parallel in the 1967 coup in Greece and while the US might have preferred NATO be a democratic bulwark that wasn't the principal objective. Indeed, Washington might have preferred a 1968 coup in Paris or Rome rather than London where the street protests were primarily about Vietnam rather than any actual counter culture. I'm going to assume the Mountbatten Government continued the policy of non-involvement in Vietnam.
The "threat" of left-wing Unions acting as a stalking horse for a left-wing Labour Government was much more pronounced in the 70s than the 60s. Let's not forget the 1983 Labour Manifesto in OTL advocated withdrawal from both NATO and the EEC - Foot may have been of that view but Wilson in the 60s wasn't - his physical decline by the mid 70s was much more apparent.
I also struggle with a British junta lasting 40 years - the last military Government in England lasted just 11 years. Authoritarianism was curiously enough on the retreat in Europe in the 70s - the regimes in Greece, Spain and Portugal had all reverted to democracies by the end of the decade and I can't see Jimmy Carter for one thinking much of the British military rulers.
The other problem the National Government will face is the one the civilian Government faced in the 1970s - the economic collapse of Butskellism following the huge hike in oil prices in 1973. Yes, you could argue the weak Labour Government was all too ready to cave in to excessive wage demands which themselves fuelled inflation but the economic response (the Howe Budget in 1981) continued and intensified the retrenchment of the public sector begun under Healey and began the decimation of manufacturing industry.
Does the Junta face down the Unions and then the rioters and is its only answer bullets? Does it have the political imagination to enact Thatcherite economic policies - I'm going to assume it does and that may well buy it some time but economic liberalisation isn't the be-all and end-all (we aren't China for example). Tianamen and the fall of the Berlin Wall will be the end of the junta and I would argue by 1990 it will be forced to re-store normal political life - perhaps an envoy from the George HW Bush administration will "persuade" London it's time for the armed forces to return to barracks.