wozza

You can buy downlight hole repair rings - search amazon / Ebay

Can you not fit the tray before building the wall?

I believe that the "Rain" sensor is built into the part with the solar panel - can you easily get to the window to test them. If I gently tap (simulate rain drops) the top part of my windows where the solar panel is they will close.

Could the bath go on the toilet wall? Then toilet and sink on the right wall - this would give window access and allow a shower above the bath.

Also consider Shelly. They now do wall switches with a plate to house the WIFI switch or dimmer - I have four Shelly Dimmer 2 and they are working well - lights are controlled by a standard wall switch (Shelly switches were not available until recently), the app or voice assistant.

They are designed to snap off with a sharp blow from the side along with the grout line (do not try against the grout line) - you should be able to see the break point. YouTube has lots of videos.

The levelling spacers are typically removed by hitting them side on with a mallet (or kicking them if on the floor)

Our extension roof (vaulted) had 100mm PIR between the joists and 75mm under. I created a service void using the MF Ceiling system (Metal frame) its really easy and something you can do yourself. We boarded over the top then used Aurora angled downlights (that can split in half to reduce the depth).

We had ours from Benchmarx, cant fault it.

You should be able to install a speed test app on your TV so can check speed easily.

I have a lot of LED downlights - I use Shelly Wifi Dimmers and a Varilight Supla Wifi Dimmer. I can control my lights with Alexa, the apps or via the simple wall switches.

We had Cemfloor. We fitted thin blue foam strips (UFH Stuff) across all doorways - no cracks apart from where it covered the foam. We were told by the contractor that we may get a cracks at two certain points (he had seen some similar floors crack and others not) and that it was up to us if we fitted foam, we didn`t fit foam and it cracked. He said it was normal and not to worry.

I don't know about the Neff hide and slide ovens, but when you open the door on our Siemens ovens the fan stops.

Using 110mm soil pipe with that extractor will be a bodge, using flexible duct of any size is also a bad idea (noisy due to turbulence from the irregular shape) IIRC my last extractor stated that the duct should be 150mm, but would allow 125mm. Have you considered using oval / rectangular duct - the high end stuff has vanes in the bends to reduce turbulence (turbulence reduces airflow and creates noise)