Big_J,
Welcome to Pprune!
You are not the first and will not be the last to consider the routes you are looking at.
And the aviation landscape has become very confusing over the years!
May I start with declaring an interest, since I own the airfield at Strathaven. I would like to think that Strathaven offers just what you are looking for: affordable and fun leisure and recreational flying. Please come and visit and have a cuppa and a biscuit and see just what is based here. You'll never see the aircraft here at Glasgow (it is £140-odd for a visitor to land there!) or at Prestwick (why bother: it costs, some are banned and there is a good pub near the Bute strip!) or even at Cumbernauld (with its c£15 landing fee)
To answer some of your questions: it is possible to swap kit-built versions of some aircraft between the microlight and light aircraft category. We had a Eurostar here that was a light aircraft based at Prestwick, then re-registered as a microlight (and therefore now banned from PIK!) Usually, as well, the microlight version of an aircraft is higher priced than the light aircraft version, so it would be unlikely to make sense to take a microlight C42 and re-register it as a light aircraft, if that was possible for that particular type.
Microlight and LAA-types are on permit to fly: so annual costs very similar. CofA is another story.
Microlight hours don't count towards keeping your EASA SEP current. Though there is talk of that changing - see what I mean about a complicated changing landscape!
What I think you need to do:
Decide if you ever want to fly at night, or IFR: not much point in Scotland in a single engine, there's almost nowhere affordable to fly to!
Decide if you might want to hire a Cessna/Piper or fly a four seater.
If so, you will need an EASA licence fairly soon.
An option: get an NPPL with a Microlight rating (if you want to fly a C42) and a SSEA rating. If you just want to fly some of the LAA light aircraft, you can skip the Micro rating.
In addition, get an EASA LAPL. (which means surrendering your SEP since you can have ony one EASA licence)
Keep your NPPL valid with 12 hours every two years: and if you have a Micro C42 then you could do 11 of those hours on that.
Do your one hour with an instructor on a light aircraft, this will keep your SSEA rating valid on your NPPL.
The EASA LAPL is valid for life, with currency determined before every flight rather that once every two years. If you don't have the required light aircraft hours to maintain LAPL currency - which you won't if you only have 1 hour SSEA and the rest microlight - then you can use your NPPL SSEA to acquire the necessary LAPL light aircraft hours!
Of course, after the "only fly EASA aircraft on EASA licence" rules come in, you would need to fly the light aircraft hours in an LAA aircraft to revalidate your LAPL.
But I would like to think you wouldn't be too bothered with EASA aircraft if you own your own.
Dare I also add, it amazes me that so few flying school operators seem to know much about all this: it is transformational stuff for private flying.