I'm thinking of applying for a PhD in Quantum Computing at Oxford. Suppose I later on decided that academia wasn't right for me. Would such a title still be useful for looking for jobs in different fields or would it be a waste of 3 years that would make me 'overqualified'?
Many types of companies look for Ph.D. holders from highly ranked institutions, even if they won't work on their research area. Think about it: this type of people have gone through a rigorous merit-based selection process; they have been trained to crack hard problems that require creative solutions; and they are used to going through a process of independent study, deep thinking, and experimenting to get results. It is not uncommon, for example, for financial or consulting firms to tap onto that kind of talent. Many R&D companies will also hire people with a variety of advanced degrees.
Maybe -- but you should certainly understand that a far more common case is to take a STEM Ph.D. and turn them into a quant, a data scientist, or possibly a computer programmer or software engineer. A business analyst or management consultant is also not unheard of (although this will probably take some passion on your part; you're less likely to find a red carpet at the end of grad school without business skills in their own right.) The financial sector loves math Ph.Ds but you might find it a bit unrelated to your super-specialty (you get to do math at least).
It's not a bad way for a career to work out, but if you're more motivated by $$ than spending several years of doing science without much $, there are much more efficient routes. That is, people who land great jobs after getting their STEM Ph.D. could very, very likely have been further along in their career if they went straight into industry. YMMV.
A PhD says you know what 'research' means -- I.e., not the undergraduate definition. That's what companies look for as opposed to a specific topic which I doubt any companies will be interested in --- a PhD is supposed to attack problems beyond the current horizon. A PhD from a reputable institution provide further evidence your research will be of high quality.