Yes, I agree with Trunks, though in practice units were dumping spare mags, tripods and barrels on Platoon transport for long distance movement. You also have the issue of it being a withdrawl under a certain amount of preasure which meant a lot of kit was being lost/dumped. You'd also ideally need early Mk.1 barrels with flared fore-ends which frankly just creates extra work for yourselves. I haven't personally seen barrels being carried in sections routinely in the way they were late war, but they quite definately were there somewhere as regs stated 2 per gun. Your also talking Mk1 Brens not Mk1M's and if you want to get REALLY anal, Mk1 mags aswell for everyone
Re: waterbottle cradles, the earliest pattern was indeed front stud, whilst I've also seen plenty of 39-40 dated top studs we have the issue of 'manufacture' compared to 'use in the field', collection are stuffed full of kit that was made in one year but just didn't make it through to use until much later, I have Mk.2 pouches for example dated 39 and 40, leather tabbed anklets dated 39 etc. and so forth, but we can only go on what actually seems to have been in use with the BEF before they decided they didn't like France and were going to come home. You could perhaps get around this by the old waterbottle in small pack sollution, but you'd probably want some people at least with them on belts just because you could.
I would personally go with the 'earlier is better' option for clothing and kit, but the question is how "right" do you want to get it for practical purposes? I'm funnily enough working out my BEF kit at the moment and the overwhelming impression you get is there's a LOT of small tricky little details that make the difference between a 'fair' and 'OK' film prop type BEF tommy and your histerically accurate type BEF Tommy, I assume as it's Mr. Heyworth then we're talking latter rather than former? There are plenty of lads that have a lot of the kit, but if your starting from scratch now then you'll need a lot of luck and money to get it balls on.
Grounsheet capes could be camo of plain early brown I think and Gas Capes are probably a very good idea worn at the alert. Final point where the f##k are people going to get pea green Blanco from!? This is in my opinion the real ba#tard to solve, mostly everything else is around or can be made, but Blanco well, that's just a giant Pea green pain in the arse.
70 miles in full kit you say? What percentage of soldiers came back in 1940 suffering complete physical and mental breakdown?