Just to echo the door point- I recently had my doors replaced, and went from having a 10mm gap under my bathroom door to a very small gap, maybe 2-3mm, and condensation has gotten noticeably worse.
That's crazy imo.
Did you put your room dimensions into the TLC vent check? I have a pretty small bathroom and I had to get an inline fan to hit their recommendation
Are they recommended when they have 1.4m of ducting to go through?
I find it very hard to believe anyone would recommend one of those extractor fans if it was doing anything but going straight through a wall.
What does the TLC fan size calculator recommend?
Aerogel is supposed to offer better performance than celotex or similar, so can be used thinner. However,
A) it's incredibly expensive
B)not sure it's suitable for floors
C)there is some scepticism as to how much better it actually is
If something is not built in line with the drawings submitted to the planning dept, they can very much take action after the fact. See Tom Moore's daughter falling foul of that in the press a few months ago.
I literally googled mixed size block paving, and this was the 2nd link.....
https://www.bartonfields.co.uk/block-paving/courtyard-tumbled-50mm-2-size-block-paving-in-graphite-blend-pack-8-35m2/
I agree with above, but you could use shims or washers under the base, then seal the gap with silicone as an alternative (unless they are really off level)