Ntplastering

Febond blue grit. apply 24hrs before wanting to skim with a brush or roller. it can be pricey but is extremely good. some paint such as kitchen paint for instance can make pva and skim very hard. the blue grit sets and give a nice key for the skim.

So depending on where you are in the country depends on the average price.
the price you have been given seems a little on the high side if i'm being honest. i would be thinking more towards the £650-£700 area for an average size room.

So first of all,you only need to fire board the ceiling.
as for plasterboard on brickwork....yes you can. just as you say,dot and dab it on. for the boiler,just work out where you would like to put it and make sure you put more dots behind the board in the area to give a solid fixing area. you dont need to insulate the walls or anything. any walls that are likely to be exposed to water(no tiles) then i would be using moisture resistant boards. all a little expensive but all very good materials.

It doesn't need to be skimmed but its generally preferred as it's another layer for strength,warmth etc... you could opt to tape and joint the wall instead but either are just as good a finish when done right.

First of all i would never skim the wall right to the bottom as it can help rising damp. i would always leave a small gap then either skirting board it or put a floor down that rolls up the wall.