From a certification viewpoint, neither category exists in the UK. You have eight categories in ascending levels of hard work to modify. These are...
1 amateur built microlights.
2 amateur built VLA category
3 type approved microlights
4 amateur built part 23 aeroplanes.
5 certified VLA category aeroplanes, annex II CofA
6 certified part 23 aeroplanes,annex II CofA
7 certified VLA category aeroplanes, EASA CofA
8 certified part 23 aeroplanes, EASA CofA
In practice no.s 1-4 will all be pretty straightforward through either BMAA or LAA, it is the certified aeroplanes that will be hard work, and if you are dealing with EASA near-impossible because they do not provide useful contact points or publish their working rules. (Mind you, nor do CAA, but they're all at Gatwick so it's reasonably easy to go and ask them directly).
There are various practices that are universal however; basically you need to demonstrate that the modifications you're making to the aeroplane comply with something called the "design code", which will vary between aircraft. This is the difficult bit since engineers who have a track record of providing this proof, and are acceptable to the various authorities, are very thin on the ground. It is wise to get such an engineer onside before you start even designing a modification, as it's generally much easier to design to comply with the design code, rather than modify something later. None of the various authorities are even faintly sympathetic to appeals to approve any modification, however worthy, if they don't comply with the design code.
If you want to drop me a PM, I may be able to provide some more detailed help - I'm one of those handful of engineers who can do this stuff and I have worked on quite a few disabled mods, and whilst normally I charge quite a lot of money for this sort of work, would normally regard disabled mods as an exception that I'd do pro-bono so long as I have the time available.
G