OSB is a really risky thing to use, especially in environments (like England) that are constantly damp. One leak and the panel is ruined. Plywood can dry out and retain its structural integrity. Plywood is generally considered the better material for home construction for all uses, walls, roofs, subfloors. I think the majority of homes are built with OSB today and I consider it the most glaring sign that the construction is cheap and to be avoided.