Apart from a registration and noise certificate, CAA absolutely will not want anything to do with your aeroplane. The whole point of SSDR, from their perspective, is to avoid any responsibility whatsoever. Ask for their opinion, and apart from those items, and whether it *appears* to fall into the SSDR category, then they won't give you one.
I'll go further, I feel the CAA do not want anything to do with anything these days, most of the recent legislation appears to be a back covering exercise to avoid getting sued, period.