We used MBC in Ireland to achieve better than .6 airtightness for our timber frame. They were half the price of the nearest equivalent (£45k for over 100sqm) and they were open to using Oak in a few key areas to avoid the 'panel' look... you should check them out.
Lovely guys, on site at the crack of dawn, very considerate and hard workers, had the frame up (actually two houses totalling over 200sqm for £90k) in ~4 days.
Good luck with planning, ours was approved on appeal (despite being in an AONB) due to debating the natural edge of the town. We then re-applied to increase the scale of the build marginally: from 2 bed to 3 bed with an extra 300mm on the roofline to enable a converted loft. Oh and we added two sheds. We had a few conditions applied, but nothing that wasn't fair.
Being on the smaller side (building something that is in high demand) seemed to get us over the line.