For Fast Track security, usually the answer is that one guest is allowed, there is a thread on this topic in the Dashboard for the exceptions - Fast Track goes under various names at different airports and sometimes the implementation is different, or occasionally there is no Fast Track for BA passengers (e.g. HEL). It's certainly OK at LHR and LGW airports for example.

For Priority Boarding it is also allowed generally, but it's not an official perk, and I guess there is a slight chance of your guest being asked to join main boarding.