The difference is that ARM is not existentially important to UK national security, whereas TSMC is existentially important to Taiwan's national security.

A TSMC fab in the US is essentially like a military base, if the US decides to take it by force the Taiwanese government can make sure there's nothing left worth taking. All of the Taiwanese employees will be loyal to Taiwan, it's their family members' lives on the line.

The only way that fab continues to operate in the event of war is if the US backs Taiwan. If Taiwan burns then that fab will go with it.