To be honest, I'm not sure there's a realistic reason for the UK to introduce a HEAT round in your timeframe - HESH is used as a gucci HE round more than an armour penetrator (certainly against other tanks) and I'm not sure HEAT is any better as a HE substitute. The L11 on the Chieftain has a series of very good APDS-T and APFDS rounds to take on T-whatevers, HESH is for BMPs and BRDMs and realistically it's more than good enough for that. If anything you'd maybe want a better HE round to take on infantry in cover, but until the L11 is found wanting against targets of that sort (which is going to be the 1990s and the Gulf War at the earliest unless 3rd Shock Army rolls) and by then the British Army is already looking at CR2 and L30 - now that's a gun that could be so much better if the UK government could occasionally put their hand in their pockets for anything other than the absolute bare minimum of R&D (top attack HE/Frag and working to improve APFSDS beyond the initial round, for example).
Thanks, that's pretty much what I suspected. I'm a big fan of Rule of Cool gadgets but in a TL sense when you start getting all the coolest, fanciest stuff you leave Earth and enter the realm of Star Wars, or CoD:IW at the very least, which is great but you then can't come back and curbstomp Argentina and Iraq with your space age superweapons.