Short answer - don't even bother trying. Buy something already approved in the UK with a G painted on the side.
Long answer - a suitable expert (and yes, I am such an expert) could do it. It would probably require a second sacrificial airframe to load test to destruction, about 4 months of the expert's time charged typically at around £50-100/hr, spread over a couple of years, a year's rent on a workshop or hangar we can put it in, and there's a high probability that you'll either fail, or need to make significant design changes to the aeroplane. You'll certainly have to completely disassemble it for inspection then rebuild it.
Not sure I've got the time, but if you are obsessed enough, drop me a line, and I can either look at the task or suggest other experts.
G