A smoke alarm that incorporates a back up power supply can be connected into a local, regularly used, lighting cct, though a dedicated supply is recommended. A local regularly used lighting cct means, practically, a cct in the same house that is used often enough for a loss of supply to it to be identified in a relatively short period of time - I don't think the interlinking issue affects this.
The RCD bit says that it should not be on an RCD, if it is, it should be on its own RCD and definitely not be on the same RCD as protects sockets liable to provide outdoor equipment. That can be a real problem in TT earthed houses that have a non-split load board (not uncommon in installations as new as, say, 6 years old!).
Ashley still needs to come clean, though, over exactly what it is he is doing.