OK, here's a rough scenario. Not that it turns the RM into the USMC, there's not enough for them.
Britain finishes WW2 with more carriers than OTL - Vanguard is completed as a third Audacious-class carrier, and some of the others are less overworked and in better condition.
The US likes the idea of another nation that can provide strike carriers, so when the cold war gets going, the US is happy to put more money into ground forces in Germany, and less into carrier aviation. The RN picks up the slack, but the BAOR is wound down to provide the budget. Lots of US tanks in Germany, but no British.
The British work with the Norwegians to cover the Northeast Atlantic, and base some aircraft on Iceland.
By 1975, the RN has 3 Audacious-class fleet carriers, 3 smaller helicopter carriers for ASW and convoy work, and 3 similar to serve as LPH. The British Army is set up as a fairly light force (very like the USMC) who expect to land in Northern Norway, hold their bases against the Soviets, and move up towards North Cape, allowing the RN and USN to push towards Murmansk.
The RAF retains UK air defence, but adds anti-shipping and long range aircraft to support the RN in the Atlantic, based out of Scotland and Iceland. They also have several wings of rough-strip aircraft to land in Norway with the Army and support them.
By 1975 the old carriers are wearing out, and new construction arrives. By 1985 the RN has 3 new fleet carriers (probably similar to CVA-01) flying Supersonic Harriers, 3 Invincible-class ASW carriers, and 3 Ocean-class LPH/LPD.
The Army doesn't have the Chieftain or Challenger, but they have a lot of helicopters, the CVR(T) family and possibly the Leopard 1. Lots of light infantry.
The RAF has Tornado ADV and antishipping, and more Supersonic Harriers to send to Norway for CAS and air defence.
If the Tornado ADV can provide air cover for the carriers (assuming tanker support), the FAA can concentrate on anti-shipping strikes.
I know this probably ignores lots of strategic realities, but history doesn't always make sense!