I've done midlands to north scotland sometimes airways and sometimes IFR outside controlled airspace (mostly, just negotiated clearance when required), and to be honest it's a lot less painful to do the latter if you carefully choose the route. As the other posters have said, the icing issue does not suit a non pressurised aircraft at all for this route. I found it much better to stay low, below the freezing layer with not too much bitting and bobbing between airspace.
edited becuase I've had a beer