EKE - don't even think about that. It is not a job for amateurs: I do that sort of certification work for a living, and would need several months, plus probably a sacrificial airframe, large workshop and a few thousands of pounds worth of working capital to certify even a simply aircraft like that through PFA - with no absolute guarantee of success.
There are loads of great microlight aeroplanes on the UK market already - X'Air,. Thruster, C42, Eurostar, Sky Ranger.... pick one, but don't make life unnecesssarily hard for yourself.
If payload is important, go for a reasonably basic engine and instrument fit, and you'll probably get the payload you want - there's always a trade-off there.
G