While ARM Holdings is based in Cambridge, England, it is majority owned by SoftBank Group which is a Japanese investment conglomerate.

What's important in the context of a European "sovereign computing" initiative is the licensing of the instruction set architecture. Like x86, ARM is still proprietary, while RISC-V is a free and open standard.

It is if you define the UK being in Europe (the continent). It isn't part of the EU as of today, although there's a good chance it will be again in the future.

Even though it's Japanese-owned most of their R&D still takes place in England.