Alternative Type 42 Batch 1 Destroyer (Or the "Big Batch 1")
That is the Batch 3 hull and machinery of OTL combined with the Batch 1 payload (except that 42 Sea Dart missiles were carried instead of 24).
24 were ordered 1968-71 in place of the 8 Type 21, 6 Type 42 Batch 1 and first 10 Leander modernisations of OTL. They were laid down 1969-74 and completed between May 1973 and the end of 1978. Each ship took about 3½ years to build.
In common with OTL the Argentine Navy bought a pair of Type 42s, but unlike OTL both of them were built in Britain.
Alternative Type 42 Batch 2 Destroyer (Or the "Big Batch 2")
This was the Batch 3 hull and machinery of OTL combined with the Batch 2 payload (except that 42 Sea Dart missiles were carried instead of 24).
In 1970 the RN had a requirement for 75 destroyers and frigates including 25 Sea Dart equipped ships. This was filled by Bristol and the 24 Type 42 Batch 1 ships built ITTL. However, ITTL the British Government offered to sell 4 of the Royal Navy's Type 42 Batch 1s to Australia after their Light Destroyer project was cancelled. The Australian Government accepted in April 1974 and the British MoD ordered 4 Type Batch 2 ships later in 1974 to replace them. These ships were laid down in 1975 and completed in 1979.
AIUI a land based version of Sea Dart (unsurprisingly known as Land Dart) was planned to replace Bloodhound in the RAF and Thunderbird in the British Army, but it was cancelled as part of the Mason defence review of 1974-75. It wasn't cancelled ITTL. Furthermore, the GWS31 version of Sea Dart survived the 1981 Defence Review of TTL.
Alternative Type 42 Batch 3 Destroyer (Or the "Big Batch 3")
This was the OTL Type 42 Batch 3 enlarged so that it could operate 2 Lynx size helicopters or one Sea King size helicopter.
IOTL 15 Sea Dart armed ships had been ordered to the end of 1978 (2 Invincibles, Bristol, 10 British Type 42 and 2 Argentine Type 42). ITTL the number was 33 (2 Invincibles, Bristol, 24 British Type 42, 4 Australian Type 42 and 2 Argentine Type 42), the TTL Type 42s carried 75% more missiles than the OTL Type 42s ordered so far and Land Dart was being built for the RAF and British Army. Therefore, the cost of Sea Dart was less than OTL due to economies of scale.
This resulted in the Argentine Government ordering 6 Type 42 Batch 3 ships from Vickers at the end of 1978 instead of the 6 MEKO 360 frigates that it bought IOTL. In common with OTL the order was reduced to 4 ships in 1980 with delivery planned for 1983-84. They were delivered to Argentina 1984-85 if the Falklands War doesn't happen and if it does happen they would be sold to Australia.
Meanwhile the Australian Government decided to buy 6 Type 42s built in Australia instead of the 6 Perry class frigates of OTL. In the end only 2 were ordered, but unlike OTL the numbers were made up by the 4 ships ordered by Argentina. Thus the Royal Australian Navy eventually acquired the 10 light destroyers that it wanted, but instead of the domestic DDL design that it wanted it received 8 British built and 2 Australian built Type 42 destroyers.
Alternative Type 22 Frigates
ITTL all Type 22s had the Batch 3 hull and machinery so that they could be armed with a single 4.5" Mk 8 gun.
16 Type 22 Batch 1 were ordered 1974-77 in place of the 4 Type 22 Batch 1, 4 Type 42 Batch 2 and second 8 Leander modernisations of OTL. They were laid down 1975-78 and completed 1979-82.
16 Type 22 Batch 2 were ordered 1978-81 in place of the 4 Type 22 Batch 2 ordered before the Falklands War, the 4 Type 42 Batch 3 and the last 2 Leander modernisations. They were laid down 1978-82 and completed 1983-86.
12 Type 22 Batch 1 would have been completed by the time the Falklands War broke out, that if it still happens ITTL, which is an improvement on the 3 that had been completed IOTL and all things being equal 8 would have taken part in the war. There would also have been 24 Type 42 destroyers, instead of 8 IOTL, but only 4 of them would have had the Type 1022 radar, although that is still an improvement on the 2 ships that had it IOTL.
IOTL 6 Type 22 frigates were ordered after the Falklands War, consisting of 2 Batch 2 and 4 Batch 3 ships. That brought the total built to 14 consisting of 4 Batch 1, 6 Batch 2 and 4 Batch 3 ships ordered 1974-85 and completed 1979-90.
ITTL a total of 16 Type 22s were ordered in the same period and all of them were built to Batch 3 standard. That brought the total to 48 consisting of 16 Batch 1, 16 Batch 2 and 16 Batch 3 ships ordered 1974-85 and completed 1979-90.
Summary
IOTL 36 destroyers and frigates were built for the RN consisting of 8 Type 21, 14 Type 22 and 14 Type 42. There were also 20 Leander class modernisations of 26 planned.
ITTL 72 destroyers and frigates were built for the RN consisting of 48 Type 22 and 24 Type 42. None of the Leander class were modernised because it was decided that building new ships was more cost effective.
Assuming that there were no losses the RN would have had 75 frigates and destroyers in 1990 consisting of Bristol, 24 Type 42, 48 Type 22 and 2 Leanders. The total IOTL was Bristol, 6 Type 21, 14 Type 22, 12 Type 42 and 18 Leander class to maintain the force of 50 destroyers and frigates that was set after the Falklands War, but the actual number was smaller than that.