Sometimes it's a good idea to try the different colour options on CAD until you find the combination that works for you. You have a good start by knowing which colours you know work well in your bedroom ... So I suggest you book some one to one time with your designer - or if you haven't got a designer consider hiring one to work with you on the project . Kind regards OnePlan Freelance Concept Planner ( yes I design kitchens too !!)

Because of the irregular shaped room I would avoid large or patterned wall tiles which will show cuts more around corners. I would choose a metro tile and paint the walls a warm but neutral colour. I do agree that pattern would be good- perhaps you could add a blind? In my opinion the layout looks good with the exception of the cabinet over the sink which looks disjointed

I;d get a lighting specialist on the case. It seems a shame to go to the expense of having ceilings lowered, when high ceilings are such a great period feature. They also allow for some of the amazing pendants, which others can't have due to the lack of height. If lighting is an issue for you, i'd solve that before embarking on the expense of lowering the ceilings and explore all options available for different areas first.

We had a similar problem. Had two rads and two plinth heaters, but they weren't working properly and we were freezing. Had the plumber in and he adjusted them and now we're toasty in a matter of minutes. Get one plinth heater put in and plug in a small electric coal or log effect stove ( cheap enough) to bridge the gap. If your windows are thermally bridged have you thought about contacting the window installer to complain , about the problems you are having? The builder, too , as he should have ensured your extension was properly insulated. The suggestion about thermal lined curtains that pull right back during the day is spot on as it will help insulate the room and the kitchen will be warmer in the mornings to come down to. Finally the door is essential as both the hall and front door area are most likely sucking away any warmth from the kitchen area, especially if there is a stairwell. I now think if I was doing mine again I would dig up the floor and put in underfoot heating but this is way too disruptive to consider now. Best of luck.