Started as a painter / decorator back in 2009 and worked for couple of companies after taking jobs for myself.
In my couple years of experience I learned other skills as well so then I started taking more complex jobs and eventually hire other people to help complete the jobs.
Then I get involved in some big projects like refurbishment on new flats .
Yes a professional job will require the skimming of the whole chimney.
Sanding down the woodwork means there is going to be dust coming on the walls also if they touch the walls with sandpaper this will require touch up which probably will be notice.
Because i assume the colour off skirting will not be white it will require two coats on it so would recommend 2.5l as it will be good to have some left for touch up later.
Painting a door with gloss paint using a roller is faster but using a quality brush it will make a better finish and it will as well keep the pattern off the door.
Wood-chip ceilings are old type off ceiling that is better to be taken off and replaced with plasterboard and then skimmed. if they are repair with bonding and then skimmed this will not be safe as it could make the ceiling collapse in future.