Hi Peter, thanks for your input.
In response to your points.
1. I'll give the bifolds some thought as you're right, they won't really be opened very often at all. We mainly just want lots of natural light entering the property. When I'm talking about alternative routes to the rear, I mean when walking between rooms. My wife could have her friends over to hang out in the open plan living space and I could be sat in the living room watching a movie. As much as I like her friends, I would prefer not to be disturbed by people walking through the living room to get to the toilet and back.
2. We asked for the ceilings to be as high as possible as we like high ceilings and want the extension to feel spacious. I think he has found a tile that works at that pitch as I remember him briefly mentioning it on a call. I will ask him as you suggest.
3. We were thinking that the office wall facing into the extension could be steel frame glazed (crittall style glass wall) which would let light in.
4. The original plan was to have a couple velux in the roof so I'm not sure why they're not there. Potentially it has to do with the pitch of the roof? I'll find out.