The economic concerns about selling off IP and industrial capacity are very valid and discussion-worthy concerns.
The defense angles seem less so; We (the USA) already supply a great deal of the warfighting technology Britain uses.
After that unfortunate tea-spilling incident and a little scuffle over taxes, our countries have been getting along like a house on fire, and there's little to no sign of that changing. I'd have no problem sourcing critical technologies and infrastructure from a British company*. This seems like a smokescreen/proxy for the _actual_ valid arguments over whether it's good for the UK economy or not (see first sentence.)
*Jokes about fire extinguishers Made In Britain aside. ;)