SSDR is Single Seat Deregulated. Its basically intended to allow competent people to design and build their own single seat microlight, however it allows anything that meets the stall speed and maximum weight requirements.
Any aeroplane produced under this regime is only inspected once when its finished. Its not subject to any design approval process. This approach clearly has pros and cons. You don't pay £2000 to the LAA to get a new type approved, and you can modify it without submitting paperwork to anybody, but on the other hand it opens the door to crazies who may build deathtraps.
There is some guidance on the LAA and CAA websites about this.