You can get fire door seals that double up as acoustic seals, whether they make much difference I don't know but I suspect they're little more (costly) than standard fire seals so probably worth a punt.
Without giving it too much thought I would look to fixing bases direct down to the concrete slab and trim the block paving around the posts. You are likely to need some kind of spacers underneath the pergola bases to get the thing level eg washers unless your concrete base is 100% level? A link...
Having a bit of a google, it looks like there were proposals made in 2015 to make them exempt from the Scottish Building Regs but I can't see any reference with regards to England Regs or whether the Scottish proposals were enacted. Bit of a funny one.
Whilst we understand and can applaud the ethos and can also sympathise, unfortunately Building Regs absolutely would apply, as would suitable insurance be necessary but if you go rogue that's your call.
At least that would be my take on it.
Congratulations on making your first post, you seem to have dragged up a year old thread and even then not realised the door is already in the open position.
Fakro, Keylite but Velux are the best and spares are usually available many years after purchase, unlike the alternatives. Have you done a search? https://www.diynot.com/diy/search/11211386/?q=velux+alternative&o=relevance
Whilst it may be obvious we're gonna start by asking what the problems are and how the ACO will rectify things. And yes you can DIY though technically you're not allowed to connect into the sewers without approval. Maybe upload a fag packet sketch of the layout and where the problems are and...
You could do it with a handsaw if you're patient and it's a half decent saw (ie not blunt), I would still fix a guide to it but I'd want to fix it to the top if using a handsaw so maybe use some clamps if you have any?
I've never done it but my concern would be the rebar starts going away from vertical as it hits/passes any roots/rocks etc and once the rebar goes wonky the sleepers won't sit square.
Yes they're not really DIYable and there is nothing like the range of finishes/colours there is for tiles, they're quite utilitarian as they're not really aimed at domestic users. A very practical solution though.