Hi,
Over the years I've built one joinery building for myself from scratch and hired a few too as well as building four of five single/double garages/workshops for others who were part time/hobbyists/DIYers etc. The caveat on this is that all of these buildings were built in New Zealand where nearly all buildings are timber frame, often timber clad, and there tends to be a lot more empty space in back yards to extend into. Carpenter/joiners are also called builder's where I come from so I am not sure if all of my advice below meets the UK regs as I've never constructed a building over here. Anyway, here's my two cents for what its worth: feel free to ignore any or all of it.
You can't beat natural light especially when sanding or finishing. Plan your window opening so it casts light onto the work bench. Unless you already have a bench to measure off decide in advance the working height you like. This way if you like your bench against the wall you could incorporate your worktop into the window sill itself. If you want the bench away from the wall still plan your window sill to this height and leave it protruding an inch or two from the wall; you'll be amazed at how often you use it to rest things on for marking out longer lengths and so on.
Now that you know this working height make sure you put a couple of 240 outlets 100mm or so above it near your work bench, your saw bench, assembly table and so on. You will trip over cords a lot less this way. If you can afford it put in as many outlets in as you can; remember also that if you may one day, or already, need a 16 amp circuit; maybe even 3 phase.
If you can line the inner walls in WBP or spruce ply. More expensive I know but you did save on the plasterer. This will give you so much scope later for hanging up jigs, blades, girly calendars etc. Also, if left unpainted you'll see wood everywhere which looks great
If you are going to have stud walls think hard in advance and get loads of noggings in at handy heights.
A great space saver that i've used a lot before is to put 300mm ply shelving around the walls that finishes 6 inches over your head so you never walk into it and you get more floor space without so many lockers/shelves etc. Being a shorta##e I could always get at least a 500mm high shelf around the entire room this way. I don't actually do this in my current joinery though as these shelves can be real dust collectors that are constantly being stirred up as doors open and close etc.
Can you get your dust collector in an adjoining box or shed outside against an exterior wall. Your neighbours might not like it but if you can get away with it you will save on space and you won't mess the place up when changing bags.
Open your exterior door outwards if you can make it work to steal a bit more space.
I know you said you want a flat roof but if you can allow yourself at least 2.5m between FFL and the ceiling you won't be knocking things over all the time when you need to end for end one of the most common timber lenghts as you can just flip it over on the spot instead of having to whir it around the round horizontally like one of those guys doing the hammer throw.
My insurance company won't let me have one where I am now but I have twice before installed small second hand wood burning stoves in the workshop. Feed them on offcuts or shavings and you won't want to leave the place in winter.
If you've got the time the main thing I'd recommend is doing all the timber work in your new workshop yourself to save money. You say you will do the lining yourself but you could also hang all the doors, do the finishings etc. Maybe even price out a timber frame building rather than blockwork. Six sections of 4m by 2.5m frame isn't too much for one guy to drag around on his own. Once the floor is done, buy a big tarp, build all the frames on the ground at your leisure covering with a tarp at the end of the day and finish with a wall raising party for all your mates. You never know one of them might show an aptitude for it. Grab him quick. He can help you pitch the roof. This is a bit challenging if you've never done it before but there must be plenty of guys on this site who'd advise you. Of course, you need to have quite a bit of spare time and energy to do all the this but at least you'll know your local inspector well by the end of it.
Good luck with it anyway.
A good size workshop at home like yours will be is every woodworkers dream.
DBC