“On national security grounds”...? Why would NVidia, a US corp, be more of a security problem than a Japanese one? If it’s just a matter of commercial overreliance on a single supplier, I find the “National security” bit to be somewhat specious.
But the British government has experience, from the Kraft purchase of Cadbury, that even if the buyers promise to keep jobs in the UK and product quality high, once the deal goes through they can renege on those promises instantly with no negative consequences.