applying for UK passport with expired US passport?

I have read the various UK gov docs and still not clear on this:

My current US passport expires in Sept this year. I will be applying for citizenship and a UK passport well before then but with the processing times it might be close as to when I get my naturalisation cert and can get my UK passport.

I will be submitting everything via a local NCS prob sometime in April.

So my question is:  Does the UK passport application require my US passport to still be valid or is it OK if it has expired?

It just needs to be valid when you apply.

As you are applying via NCS, you'll hold onto your US passport and can renew it before it expires.