Like most other countries the British Armed Services fight amongst themselves for slices of the budget with a viciousness that if turned on the enemy would make them feared worldwide.
But what if after WW2 they realised that the slice is getting ever smaller and decided to cooperate whenever possible, what could have been achieved?
The most obvious to me is the Javelin and Sea Vixen, everything duplicated for aircraft that do the same job. It doesn't have to be complete projects, perhaps the RN is interested in a piece of avionics the RAF is developing and kicks the can to get it over the line for example.