Mr Punter

If it looks OK then it is OK. I have a property where the floor slopes 50mm over 1200mm. You only really notice when you look at the plinth.

I understand that Sweet Chestnut is a good durable UK timber.

If you are on clay soil you would need to excavate a vast amount and backfill with the granular permeable base material. You may need to go down half a metre from the finish level. I have laid permeable paving on a fairly thin bed of granite chippings on chalk subgrade and it drains very well.

The boreholes should not be too costly. If they are good designers they may use the information from the geotechnical report for a more cost effective solution for the pool.

Another vote for adding more battens. Just remove any that really mess up your gauge. The extra battens will strengthen the roof a bit. Could you get different shingles to work?

Well you could have a quick read of:

Estimators online are OK. You will need to send a full set of drawings and spec.

You can fit the frame into the timber studwork.

Whoops!

If you go wall hung, get the pan and frame on site before you set up the pipework, as the height of the flush pipe and pan connector are important.

Only the timber cladding would be a concern, as well as the "composite" if it is flammable, especially if the breather paper and sheathing are also combustible.

With timber frame and timber cladding you will not get decent fire rating without further protection such as boards or coatings.

For brands, Dewalt and Makita are good but cheaper brands like Erbauer and Titan can perform well. You will need more than one decent capacity battery and make sure it will fit any other tools you are considering. Go for 18V and batteries at least 3.0Ah. If you get stuff from Screwfix or Toolstation it is easy to return them if they fail.

spirit level accuracy not required but obvs the fall must be in the correct direction.

Be interesting to see the as-built version of the drawings. That is a lot of different products being applied at the substructure level. The insulation could be held in place with wall ties, although you could just use blown beads.