Hmm, I think we might have gone way off topic here. I suspect you might have been better off posting in "Plastering & Rendering" or even in "Floors, Stairs & Lofts"
A long post so bear with me.
Correct me if I'm wrong. You do not intend to remove any timber. You just want to remove the existing ceiling plaster board (shown in your pic 3) and replace it, but a little higher up
between the ceing joists ( as shown in my drawing
View media item 17596
so as to create an exposed beam effect? In which case your roof/ceiling construction is not affected.
If so, it is perfectly acceptable, construction wise.
If I were you I would obtain an estimate first in order to weigh up the cost compared to the aesthetical benefits. (You can possibly just skim over the artex. Ask on Plastering & Rendering forum.)
The newly exposed joists will require some work because they'll be rough and dirty. It's also going to be a very dirty, dusty job so you'll need to isolate the room by covering doorways, etc, but give some thought to how you'll remove all the debris from the room.
Also, the loft insulation (the messy fibre stuff) will fall down and need to be replaced but I suspect that you ought to have more anyway.
OK, Big question: are you intending this as a DIY? There are some jobs easily within a DIY scope and some not so easy.
If DIY then:
Decide if you're going to remove the insulation and plasterboard(PB) from above or remove the PB from below and let the insulation fall down. PB is fairly easy to remove with a hammer or a foot from above and possibly cleaner for the operator. But take care you don't fall through!
Remove insulation and PB.
Watch out for pipes & cables.
De-nail joists, especially bottoms and sides. Tops of joists should already be reasonably clear.
Fix new timber to sides of joists as shown in my drawing. I would suggest 50mm X 25mm but you ought to ask on Plastering & Rendering forum. Again
watch out for pipes & cables, if necc move them higher, above the new timber.
Then decide how you're going to insulate from above or from below because there isn't much room up there. If from below, before fixing PB, you'll need to think about rigid insulation. From above, of course, after fixing PB you'll be using mineralwool or similar. Check this out & then decide if you're DIY'ing
http://www.cus.net/insulation/subcats/loftinsulationhowto.html
Pay special attention to eaves with the insulation. You need to maintain ventilation at eaves into the loft space. (Ask in Floors, Stairs & Lofts)
Fix appropriate PB and finish accordingly (Yes, you've guessed it...ask in Plastering & Rendering)
