Unfortunately this country is so highly regulated in the aviation business that being legal is very difficult.
Air law. You are not allowed to be closer than 500feet from "vessels, vehicles, structures or people".
Controlled airspace. Class D around airfields often extends from the surface to several thousand feet and you need permission to enter that airspace.
You also need the land owners permission to operate from wherever you wish to operate from.
So much for the depressing bit.
There is the "single seat de-regulated" (SSDR) class of aircraft, limited to some weight, stall speed etc, can't remember details, it was115kg max but has since been increased somewhat. Design, build as you wish, unregulated. However you still need a licence to fly it along with following the rules/law.
I don't think you can avoid some form of basic microlight pilots licence to remain legal, though once you have that you will be able to launch from recognised airstrips with the appropriate permission.
Welcome to the world of hassle, endless rules and regulations.
Best of luck with your project.
I have had similar thoughts.