We have raked ceilings in our new split level extension that follows the fall of the land (and in the rooms that were part of the skillion sections of the old cottage) and I love the sense of space they give and also the fact that the inside reflects what is happening on the outside of the house. We have used ceiling boards (painted white – I didn't want to feel like I was living in a cigar box) both in the old cottage and in the new section and they certainly add an enormous amount of visual interest to the rooms as well. However, I endorse the comments about the added trickiness of light fittings. Pendants with rods are out and pay particular attention to the little rose/canopy thingies that connect pendants to the ceiling – make sure that they sit flush with the ceiling and don't have weird unsightly bits that you can suddenly see because of the angled ceiling. We have successfully used LED downlights with a recessed diffuser and dimmer switches to make sure there is no glare but if the raked angle isn't too extreme they are a definite possibility.

For a "couple of days a year" get a pedestal fan straight on everyone and leave the features in the ceiling. Seriously...I have 12 ceiling fans in my house and two wall mounted fans...the wall mounted fans are so much more effective then ceiling (and I don't know where you live, but did you know fans have a "summer" and "winter" setting??). Most people don't. Tip: turn the fan off before changing the setting.

To assist with harnessing the sea breeze suggest a beautiful long narrow feature window that follows either side of the pitch of the roof (which will let in a lovely sky view plus breeze), or just have the top triangle as a window. Understand the desire to have the small window there however in a room with big glass/window features, the small window looks like an after thought and is at the wrong scale.

You never mentioned your budget that you're willing to spend. You could add a thicker fill backed insulation blanket - 50mm, 75mm or 100mm are the common thicknesses. Steep roof pitch so would need scaffolding, harnesses, etc.. which adds up quickly. Probably still not enough insulation though but probably cheapest option. Insulation under existing timber linings then reline over the top of that as some have suggested. Working at height though so would need scaffold set up again and working overhead is not fun nor easy. You could replace the roof with a sandwich panel roof. Try Versiclad or Kingspan or comparable. They have a polystyrene type of insulation between an outer metal roof sheet and a metal inner sheet. These are heat laminated together. Again different thickness available from 50mm up to 180mm is common. The 180mm has a R rating of 4.7 which is quite good. Again steep pitch, etc., probably need new ridge capping, etc.. Probably most expensive option...... Keep us all posted on your decision though!