Had a colleague who used the fly the shed for British Midland and he recounted a tale of flying from Heathrow to Brum one winters evening and starting out flight planned at something like FL80 but being down at 60 by WCO and dropping out of controlled airspace with the speed slowly decaying sometime later. Made it to BHX but he wasn't impressed to say the least. He called them flying ice cubes.
This video gives an idea. ( albeit from across the pond) of how icing built up on the 360