if I have a company that has a registered office (not a real one) in London, is it ok to use that address as the one for my domain name?
It depends on which organisation is ultimately responsible for the top level domain, but for .com, .org and others, it's ICANN. They say
Your domain name registration agreement with your registrar requires that you keep this information accurate and current.
...it is available to be searched by members of the public in order to allow rapid resolution of technical problems and to permit enforcement of consumer protection, trademark, and other laws
So the address is OK if someone can get hold of you at that address for the reasons mentioned there.