most codes say 30" to combustible surface like a decorative wood hood over cooktop. the metal style hoods can be a little less but keep in mind your eye level & also how far the hood projects as you don't want to get bopped in the head every time you are cooking. the typical 6" high hood are generally 63" to underside with overhead cabinet about 69" floor to underside.

I had a similar situation in a house I moved into years ago. I did a quick fix of painting all of the walls, woodword and the cupboards the same shade of cream. Sounds a bit OTT but it really knocked back the cupboards, which in all honesty are not something you want to make a feature out of, and created a calmness to the whole space. In your case, I would go for an off white so that the gang plug wires, radiator pipes and covers hiding the wires for the extractor fan etc., are disguised as much as possible -- that will tie in with the palest of the tiles in the splash back, and then you will have a pretty basic colour scheme of a very place colour and the dark worktop, fireplace and floor, which will stand out more. I think you need more storage with doors -- that bookshelf isn't doing anything for the kitchen. If you could get something in there that could hide lots of clutter -- even if just something cheap from IKEA, that would help greatly. Or work something around the filing cabinet that you have? A huge cheap mirror over the fireplace would bounce some light around too. If you don't want to go for undercupboard lighting, and if you have enough plug points, how about some lamps on the worktop, especially at the FF end with a floor standing lamp at the end with the chair. For the table, I would go for a table cloth and reupholster the chairs if they are not to your taste. A new blind in a much fresher colour would be great. Depending on the colour of your filing cabinet (if you use it), I think a petrol blue could add a nice vintage vibe to the chimney breast, and make more of a feature of it. I would try and go down a vintage/slightly industrial route rather than country. You may have some stuff lurking around that you could use, once you have got the base colours right -- currently it just feels a little cluttered and a bit of a mish mash. Best of luck. Looking forward to seeing some after shots :)

In my opinion an extractor is invaluable to limit cooking smells in your home. You could put the hob under the extractor vent. You could put industrial style overhead ducting to connect an extractor between the windows to the vent. You could have a deeper worktop and a pop up extractor. You could continue the boxing for the extractor vent above the windows to allow you to have an extractor elsewhere. This is what I would do. I would also consider if it is better to put the kitchen in the left hand corner where there are no windows and to move the sink to allow a dining area close to the main door.

white is probabbly the worst colour for changing colour (yellowing due to UV) over time, if you're painting the decor panels, you'll need the paint code (ral, pantone, BS number etc) used for the doors. also i'd find out the primer being used as well. also find out the level of sheen the doors are. but TBH with a mass produced off the shelf kitchen supplier customers don't get a kitchen that has been painted all at the same time and possibly not all painted in the same factory, its quite common that one factory makes the doors then another factory makes accessories. also if the doors aren't painted but a vinyl foil then a painted panel isn't going to match anyway.