Not sure if this is incongruent with your sizing information, but I have bought size XL shirts and outerwear in Japan. Fits like typical large (I am a 42" chest) but somewhat short in the body and arms. I am however unsure whether Western sizing Margaret Howell is also short in length, so that might not be Japan specific.
MH UK/Europe is not short in length. The Japanese line is made specifically for the Japanese market and is sized and cut to reflect this. General rule of thumb is a Japanese XL is the equivilant of a Large, but as you note the sleeves are cut shorter. I was looking at a shirt on the Japanses site a couple of days ago and based on the measurements a UK Large would be a XXL. Although most of the Japanese collection is made in Japan some of the garments appear to be made in the UK, scarves and Mainline knitwear appear to be made in the UK but I don't know if they change the sizing for knitwear or if it's the same stock as we get in the UK? Some Japanese made garments and accesorries end up on sell here, this T for example: https://www.margarethowell.co.uk/men-aw16/sportswear/mhl-roll-sleeve-tshirt-japanese-cotton-blue MHL sweatshirts were made by Loopwheeler and the bags are made by Yoshida.
Edited by ike_hiking_boots - 8/25/16 at 1:43pm