A US trade deal is a symbolic and political victory but makes virtually no difference economically. I don’t think the UK government will calculate that prospect of one would outweigh losing ARM.
Faced with the political impact of "that amazing US trade deal we promised you is off the table" versus "a company you probably haven't actually heard of, which was already Japanese-owned, is becoming US-owned"… well, I don't have much confidence.
(Is it still true the UK-US trade deal is awaiting elections in the latter? EU-UK has certainly not been progressing.)