Good plans those Ian!
The reason I have used a letter for the thickness is precisely because of the apparently random nature of the thickness of conti in various places.
I have found that although Wickes is cheaper, the conti there is still 15mm and my local Wickes doesn't cut to size. Homebase is also 15mm. B&Q does 18mm Conti now, and in my experience it is in much better condition than that at either of the other two (less battered). I also find B&Q to have a greater range of sizes in each of the different colours. (others may well have different experiences)
I have always had great success with B&Q and they cut it accurately aswell - TOP TIP - get them to square off the end of each sheet before they cut the main pieces!
Good luck with the projects folks