Lima Juliet,
Because it is a silly comparison. Australia does not have two SSBN nor three and a half SSN, nor does it have one 65,0000 ton aircraft carrier or 38 Chinooks or 25 Apaches.
The RAF has never operated dedicated SEAD/DEAD/EA aircraft, and even during the Cold War neither did most of NATO, it was just a niche capability provided in small numbers by the US and the Germans. And that niche capability never existed in numbers to enable it to be everywhere at once, so it was never seen as a must have by anybody.
Asturia56,
A national capability in certain defence manufacturing is seen by the UK as an essential national asset and capability, if you buy everything from overseas then you risk your foreign policy inevitably becoming dependent upon those you buy from, not very independent and not a very good example of "taking back control". You would also ruin the second largest defence and aerospace industry in the world with all the implications for employment, GDP and tax take.