I reckon 28 days is indeed a bit steep to get a AFD system & EL fitted. This leaves next to no time for the RP to shop around and get various quotes in, which I reckon is unreasonable.
As far as suggesting the RP should wait for the FRA to suggest that an AFD system is required, I am unsure about this. It will be obvious - even for us lot who have hardly any knowledge of this building- that some kind of detection & EL will be required.
If I were writing such a notice, I too would include the main 'headlines' - the most important requirements- on the schedule and leave the findings of the FRA to mop up all the other stuff.
If not included on the notice, there is a possibility of further delays installing a system. I would perhaps give three months or so to do the lot as a reasonable time scale.
In the event that the work is not completed on time, it would be very difficult for the RP to argue that the notice was unreasonable or there wasn't enough time