The 'what if the British military had acted in 1965 to arrest Ian Smith and stop UDI' issue has been dealt with , in a slightly different way of action after the UDI declaration, in the now forgotten alt hist novel by Peter Van Greenaway published in 1968, The Man Who Held the Queen To Ransom And Sent Parliament Packing (Weidenfeld and Nicolson publishers). I read this when I was about eleven and found it gripping; it has a 'What If' of a group of idealistic young left-wing but not specifically Marxist British military officers, with a mixture of admiration of Castro-style armed revolutionaries, black African junior officer coups, and the Levellers and Cromwell in the UK, staging a coup in Westminster in 1967 .
They march into Parliament to re-enact Cromwell's expulsion of the Rump Parliament in 1653 by throwing out the MPs and Speaker at gunpoint, seize the Queen and Royal Family at Buckingham Palace and hold them hostage at the Tower of London to keep the establishment and its forces at bay, denounce the US and CIA and call the Cold War a sham propagated by the military-industrial complex, and appeal to the 'anti-declinist' frustrations of the public after the end of Empire by blaming it all on the elite's greed and complacency in propaganda speeches and revelations of their shabby financial deals and feather-bedded incomes on the rebel-seized BBC, reading out confiscated documents and private letters in public. The rebels, a mixture of junior officers and civilian allies from the non-party left, are shown as being honest but naive - and they have a vague Kennedyesque 'Young Turk' reformist dislike of the 'Old Guard' 1950s elite influenced by US liberal writers and thinkers which includes anti-racism . The refusal of the devious supposedly left-wing but really power-hoarding and cynical Labour PM, a thinly disguised portrait of Harold Wilson the then PM (who is supposed to have faced coup threats for not doing more to reverse decline in 1968 OTL), to do anything about UDI in Rhodesia for fear of right wing army officers and racist soldiers refusing to fight their 'kith and kin' is the event that pushes them over the brink into revolt, and they send in a force of more disciplined elite paratroopers by air to Zambia in semi-secret under a news blackout to go on to Salisbury/ Harare and arrest Smith and his govt. The Conservatives are seen as a hopeless opposition, too scared of their own Right wing and the feral press (and with their own imperialists who regret African independence) to do anything -and a number of their senior figures despise the weak and timid Con leader (a version of OTL leader Ted Heath) and are crooked business tycoons who fleece the govt and the system for profit and are linked to belligerent anti-democratic MI5 'blimps' and Bond-style, US-allied right-wingers who admire Ian Smith.
There is little resistance to the landing in Rhodesia as it is a surprise to the Rhodesians who expected the British govt to be useless and the Brit army to refuse to fight them, and the forced announcement of quick elections and a black majority govt is greeted with cheers by the Southern African black nations and the Commonwealth as a whole - and restores Britain's battered image for democracy for all races not just whites. The riots and angry protests are in the UK, stirred up by the equivalent of Enoch Powell and funded by the furious CIA and Pres Johnson as the new UK govt is threatening to leave NATO so it is 'clearly a Soviet lackey' (which it isn't). The US backs and a secret cabal of MI5 right-wingers organises a counter-coup and the rescue of the Royals, and army units loyal to the affronted right wing senior generals surround and storm the rebels' HQ in Parliament; the arrested rebel leader is to be tried for 'high treason' in a show trial later on TV but the coup leaders fear he will reveal more of their secret documents' contents in court so they bribe a low-life racist gangster who's been sprung from prison to assassinate him on his way into court, Lee Harvey Oswald-style. The assassin has been promised to be secretly got out of prison in disguise and smuggled into exile with a new identity later - but the book ends with him being cynically shot moments after the assassination by M15 agents to keep him quiet.
The mixture of cynical Establishment manoevures and secret M15 and CIA control of the course of events and of the greedy politicians was a forerunner of the Chris Mullin novels on a similar theme, and very 'Watergate era' - but written years earlier. The monarchy is shown as a puppet providing entertainment and gossip to keep the 'bread and circuses' public quiet, and the senior civil servants as manipulating the feckless, constantly plotting, status-obsessed ministers who ignore all attempts at or calls for reform - with the police and MI5 lurking in the background obsessed with subversion. The comment of the chief of the pro-CIA civilian cabal on the intention of the counter-coup serves to sum up much about modern politics - they've put the electorate (who the rebels hoped to inspire to be more active) back where it belongs, caught between the left and the right of the political nutcracker.