If you're a "normal build", then you (in my experience) should order a size up. If you normally wear a 42 Reg in a suit jacket size (and it fits fine in the body and shoulders), order a 44 Reg in Kilt Jacket from the UK.

The tricky part is when you are a bit wisproportioned... if you have a VERY large belly or have a VERY athletic cut (6 pack abs and broad chest). Then you may need to get a jacket tailored to your exact measurements so it will either close in the front (if you're big) or won't be too baggy in the front (if you're athletic).