That may be true but, whatever, if he was telling us the truth about the FCU being a spur off the ring, then he would not have had that option open to him, anyway.
They would not have an option to take a second spur, is that what you mean?
But I would also be concerned that the OP does not know the current load rating of the macerator pump or took the trouble to find out! and also is considering taking a 3A feed to 13A sockets and installing this in flex.
Does not sound to me, as being one of any respectable electrical background knowledge.