Yes, every First passenger can bring in another passenger regardless of class, status or flight time, as long as they are on a same-day BA flight.

The Concorde Dining Room within the Concorde Room at T5 is open all day, with waiter service from breakfast to dinner. I have had breakfast there, which was very good but the service was a little slow with some clueless waiters.

See my Part 1 trip report below for pictures;