By UK law that aeroplane will still require a registration and noise certificate, and the pilot will require a licence.
By Darwin's law, the designer/builders will involve some proper aeronautical grown-ups, such as may be found within the BMAA and LAA; at that weight range, I'd go to
BMAA first.
Ladders are lightweight safety critical structures that must comply with some pretty stringent EU regulations, and so are almost certainly an excellent "building block" for making a simple lightweight aeroplane out of.
Again, Darwin's law - have it test flown by somebody who knows about test flying little aeroplanes. There is a
reasonable community of people in the UK with these skills. Subsequently, training in a tailwheel Thruster would be pretty appropriate.
So, I'd absolutely say go for it, just make two points:
- Get help from people with significant prior little aeroplane design/build/test experience
- If you are an aero-eng undergrad, see if you can find a way to make this your dissertation project!
I'd give you a hand, looks tremendous fun - the little ladder-built single anyhow; I can't honestly say I know enough about paramotors to trust 'em. Incidentally your second link doesn't work, but the pictures to me look like this is modelled very closely upon the Mignet HM293, which I have flown and enjoyed enormously.
G