Isnt building a joint missile sub based on two completely different missile systems, both in size and associated systems going to make any potential cost saving disappear. What hull dimensions do you use, what are the power requirements etc.? You cant just mix and match on a sub, we dont need a joint venture, we have a plan if needed, we are likely to adapt an Astute to fit a few Trident tubes with a hump or extended sail, and wont design anything until we know what the dimensions of Trident E6 will be.
Competing to sell Typhoons in the MRCA competition is one thing (a big IF to win it) with maybe sized or tech down AESA etc, but fighters are not in the league of an SSN, a top class SSN wins a conflict, or in the hands of an enemy could force us to lose a conflict or not engage in a conflict. I recall the C5 programme on the Ark Royal last year , the T class boat commander was suggesting He had to "let" the Merlins find him during an excercise, the capabilities are that good. The Astute is as good as we can get, not quite maybe as good as the USA, but good enough to keep our Navy, with small numbers in the lead. Selling it, or giving the tech to India is IMO about as bad an idea as you can get. Market subystems etc, some aspects (but not your cutting edge secrets).
In the coming decades India will strive to become dominant in its region, our policy goals might not match the UK's
A further point, the UK gives over a three year period 850m in aid to India, I know this is linked to trade deals etc,but India can afford carriers, SU30MKI's and nuclear weapons etc and I suspect 850m would pay quite nicely for an extra RN Astute. We should not fund Indias poor, they should.
The critical state of our sub industry is a political choice and mismangment, there are ample funds to pay for the Navy if the politcal will was there. 0.25% of GDP would solve many problems. The Aussies I would trust, the Indians Im afraid I wouldnt, the nuclear standoff with Pakistan is always going to bubble.
Prostituting our remaining advantage IMO is mad, but I have no tech knowlege, I would be interested to see how many of the more in the know posters on here would agree with you.