As long as the T&G exterior is OK, the BillyOh would probably work if you are prepared to beef-up the framing a bit. You might want to do that to increase the thickness of the insulation anyway. The biggest drawback of the BillyOh might be the need to refinish the exterior every year. Unless you happen to love painting, that can get old really fast.
I don't know if there is an equivalent product in the UK, but there is fiber-cement siding (Hardieplank) available in the US. Zero maintenance, and it looks great.
Probably not appropriate for your location, but I cobbled this together. It's a knock-off of a kit supplied by a local building supply company. The framing is all "2x4" (really 1.5 x 3.5) and the exterior is hard siding. It was only supposed to last a few years and that was twenty years ago (note moss on roof). It has had absolutely no maintenance. In fact, there are a couple of bits that I never really finished, but I'm going to get around to them any day now.
I'm not sure why, but for some reason lumber is really cheap around here.