Oh nice one: I was still dithering about how to best to split this discussion off when you posted....
Here's a scheme along the same lines but probably cheaper and quicker.
1. Build three new "pure" harrier-carriers of about 30,000 tons, each carrying 24 x SHARs, 4 x AEW Sea Kings and two SAR/plane-guard Sea Kings.
2. Keep the Invincibles, but use them as pure helo carriers with Sea Dart removed and carrying 18+ ASW Sea Kings.
3. Build at least eight new high-end escort cruisers based on the abortive Type 43 design but with AEGIS and Mk.41 VLS. WHAT? and this from a Sea Dart fan?!!! Well yes, but I'm being realistic: nothing else will do the job in a reasonable timeframe. Any VLS Sea Dart development would be pretty much starting from scratch and in the mid 80s, ASTER was decades away and on the wrong side of a lot of politics. D.K. Brown, who designed the Type 43, has stated in his book that they did a study on fitting AEGIS and it was both feasible and better than double-ended Sea Dart.
One thing I wouldn't carry over from the Type 43 is the mid-mounted helo deck. Even Brown, who designed it, had to admit that airmen wern't happy with it and I suspect that's something of an understatement. I'd use the space for Harpoon and Containerised Ikara (real project: got as far as test-firings), slide the after superstructure forwards and fit a helo deck, not neccessarily with a hangar, at the stern. What might be feasible is a Merlin/Sea-King-sized deck with a Lynx hangar, the Lynx being their more for anti-FPB patrol than ASW.
4. The Type 42s can soldier on (with upgrades) while this new construction takes place, to eventully be replaced by the results of the NATO NF90 common frigate programme (ASTER-armed DDG in essence). Since the RN would already have the high-end CGs by the time this came around, there'd be less of the pressure from them to make NF90 super-capable that led to the break up of the project in the real world.
5. The Type 23s can be pretty much left alone. Their main mission is towed array work which puts them a long way from friendly cover (to find quiet water) thus leaving them quite exposed, which is why they went from "austere tugs" to full-on frigates in the design process. Once you've got a lowish-draught ship with a decent point-defence system and a helo, it then becomes a natural plaform for a medium calibre gun for NGS work.
One developmental item I'd like to add here (not too expensive I think) is to build some or all of the Type 23s with the OTO-Melara 5"/54 in place of the 4.5" Mk.8, possibly with a Vickers GRP-composite turret shell to make it lighter. Why? Well this is a FAR better gun than the Mk.8 and it uses US (and therefore effectively NATO) standard 5" ammo, so it can take advantage of ALL future US developments. This would be the first step towards shifting the whole RN over to 5", the next one being NF90.
6. I can see the argument for a low-cost, unprovocative "sloop" for peacetime, low-threat environments (West Indies guard ship, for instance), but I think it needs to be VERY low-end to be cost effective: more like a "global OPV" than a light frigate. Brown makes the point that, for a given threat, there are cost-range "holes" where a ship is too cheap to defend itself but too expensive to lose where it's near-impossible to design something worthwhile. Something like the Danish Thetis class would be ideal:
https://en.wikipedia.org/wiki/Thetis-class_ocean_patrol_vessel7. I agree about multi-role LHDs insted of the dated LPD/LPH concept we have now. Three vessels similar to the French Mistrals or the new Spanish ships instead of Albion/Bulwark/Ocean would be ideal.
8. Lastly, if we're committed to STOVL carrier aviation and we've got money to burn, how about developing the P.1216 as a Sea Harrier replacement?