If you contact your local council, they'll be able to tell you.

You can give it a go, the sheds like wickes, screwfix, howdens etc have a limited range of colorfill to choose from, but its not something i've ever had to do so i don't know how well paint adheres to it, i suggested colorfill as it is in a wide range of colours (on line search). Decorators caulk (comes in a tube and applied like silicon with a mastic gun) though is designed to be painted on, so if you have a colour match paint then the caulk might be a better option.

I’m going to be stupid here and say are you sure the doors are supposed to be the same as the board behind? Perhaps the previous owners wanted a contrast. I can’t see a specific door that shows a stain as such. In your favour the doors are cut and edge board so are the cheapest you can get. The handles are boss bar handles and again one of the cheapest handles. You could remove the handles, get new doors and add the existing handles. Cut and edged board has 100s of colour variations, so plenty of choice. You could even have a multiplex edging to give a more luxurious touch (looks like a ply timber edging and gives a nice finish).

Option 1 seems easiest and if it gives you enough space for the kitchen you want, then go for that. Having the bigger utility in the garage conversion is great too, very useful. You could make that Study external door into a window quite easily (or just lock the door and never use it - for someone who works from home and has clients coming to the house, this would be an ideal set-up though) You could add a door into the new utility instead of just having windows there, then you've got the external access into utility room to contain shoes/coats/mess. My first thought was also how many living rooms do you need? But big formal adult/family room, snug attached to kitchen for everyday use, and kids playroom or teens hangout-space or cinema-room for the garage conversion - actually it's perfect if you have more than one kid (although the big formal one will get the least use, unless the kids prefer it). I'd make sure there was a door between big living room and the snug though, no need for that to be open to each other.