Not until they decide to go public and list their ordinary shares in a recognised exchange.
Technically ARM Holding is still a private company with no public share offerings. Their listing is on Nasdaq is an ADR. Basically its a “receipt” for a right to the value of “certain number of” stocks. You can google the difference between a stock and an ADR (or ADS in a similar manner)
you can buy it in invest, or sipp not in isa.
edit: I was wondering “this must have been asked before” and you asked the exact same thing few months back…