Great boots, keep me toasty. They aren't particularly deep, but are suitably wide for British feet. I bought the same size as I do for most wide-ish footwear a UK 12. due to the felt liners I don't go overboard on socks just wearing a standard single thickish winter pair. If you go too thick on the socks getting your foot with the inner on inside the boot can be a PITA.
As I recall I simply ordered my normal size and the boot came suitably proportioned to accommodate, without excessive slop, the full height pull-on liners that can be worn as waterproof slippers and are great for attending to a call of nature when under canvas. The model in question is not however the now fashionable pack boot with wool or felt calf trim but rather a super-rugged leather hunting boot:
Needles to say they don't get much use in Lowland Britain!