I think the quick answer is that it's very important but not absolutely important.
The U.K. wants to present itself as an operationally independent major international power. They recognize this means that you cannot depend entirely or excessively on a foreign supplier. I think Ukraine has brought out the importance of supply chains in the defence capability. There's a variety of reasons for this, in part to do with prosperity and these other things.
There is government documentation, particularly the defence and security industrial strategy announced in 2021, explaining why the government concluded that this was important and what they were going to try to maintain. Then we have quite a series of sector strategies. I think it's very much down to this. A document in 2012 said that the ability to use your armed forces as you see fit is the essence of sovereignty. Now, that raises all kinds of questions about what sovereignty means, but it says that it's something quite important—