The CWU-45/P is a nice jacket, but waaaay too short. Elastic knit waist rides up too much. That, and it soaks up water like a sponge.
Highfinal: nope, noone on this side of the pond carries the Mk3. Apparently Alpha made a copy, but I've never been able to find reference from them, only the occasional rare listing on ebay.
Another option is the Canadian Mk4 winter-weight, which has basically the same cut as Brit Mk3. Much, much warmer too. It does, however, have a natty fake-real fur collar (which can be removed on the newer version. Great piece of kit. With a new gore-tex/nomex version being issued, the original cotton/wool ones are becoming readily available, just check out ebay.
For southern climbs heliduck, the Canuck intermediate jacket is excellent. We call it intermediate, but its about the same weight as the brit Mk3. Velcro tabs on the waist, not elastic, keeps the jacket from riding up. Long enough so it covers the kidneys nicely. For those who like extra pockets, it can't be beat. Two on each arm, one upper, one lower. Two inner pockets, plus two front slash pockets. Apparently, surplus guys here sell quite a few to helo types in Hawaii!
One guy on Ebay sells alot, though he has nothing up at the mo'. Just look for "aa-surplus", or search "canadian military".
MKIV from ebay
Lightweight (color on surplus are normally AF blue, like buddies wedge cap).