Can't see how this has anything to do with Brexit
ARM is an attractive investment, regardless of where it's based, and regardless of the outcome of the referendum.
And as for ARM being a 'strategic asset' which the government should 'protect', what does that really mean? The government has a poor track record of such a thing - usually the industries it 'protects' end up being sclerotic and dysfunctional.
The things we in the UK rely on - power, food, and, yes, electronics - all critical to our survival and supposedly needing of 'strategic protection' mostly come from abroad. For better or worse (I think better), we're living in an interconnected world - very much the world that ARM has flourished in.
Luckily for us, the politicians aren't very good at spotting and 'protecting' (i.e. suffocating) the next ARM.