All right the consensus is that even medium sized carriers are probably more that the treasury is willing to pay for. Lets go the other way while still keeping a conventional carrier capability.
For a small conventional carrier HMS Victorious was a good ship, able to carry a reasonably ballanced air group. By the time of the 1970 election she is however already at the breakers. Lets say that instead of the OTL Invincible class the RN manages to persuade the government to order 3 ships roughly similar to the old Victorious but taking advantage of improved technology since she was rebuilt. According to my books Victorious could carry an airgroup of 36, including Buccaneers, Sea Vixans, Gannets and ASW/SAR helicopters. The Navy would insist on retaining the Buccaneers but an alternative to the Phantom would have to be found and the Sea Vixan was by now verging on obsolete. They will also need an AEW cabability.
For AEW while the Gannet can soldier on for a few more years the airframe hours are climbing and the radar is pretty long in the tooth.
Options.
Helicopter, it works but not ideal.
New Build Gannets, Probably not feasable. The production line is long gone and would be very expensive to reconstitute.
Rebuild old Gannets. Assumes airframes worth rebuilding, still need new radar.
Joint programme with France for new AEW aircraft. Expensive and joint projects have a habit of escalating in price.
Surplus E1 Tracer or rebuilt S2 Tracker, prefeably with new turbo prop engines. Plenty of Airframes and spares and probably the most capable and cost effective option. Drawback is it's a relatively large airframe. Could also be used for the RAF's AEW needs.
Fighter.
Options.
F8 Crusader. Old design, day fighter and only one engine. Available from USN reserve aircraft, proven design comonality with the French carriers.
Two seat Jaguar M with the radar fited OTL to the Sea Harrier. Not really a fighter, but has two engines and can carry missiles with the Sea Harriers Radar and a back seater should be adequate just.
Wait for F18. No garuntee it will fit on the new ships.
Develope new carrier fighter. No far too expensive with no export market.
Sugested air group for the ships.
8 Buccaneers 12 in emergency.
8 F8 Crusaders 12 in emergency.
4 AEW S2 Tracker rebuilds.
10 ASW/SAR Helicopters Wessex to begin with then either SeaKings or Lynx (all things equal I'd go for the Lynx to save space, the Navy would probably opt for Seaking)
Buccs and F8s would probably be replaced with either F/A 18s or Rafales.
I would recommend deck edge lifts to maximise hanger space so that fixed wing combat aircraft could be boosted to a maximum of 30 for short periods with 20 being the normal complement.