These guys make external windshield covers and other accessories for heavy snow and ice.
and of course there's Amazon. Amazon UK options - You may be able to find these available off of amazon
Personally I prefer a custom fitted internal sun shield in the summer, and a Snow Joe for the winter. The heatshield sunshield is reversable, so in winter you can have the dark side face outwards to absorb heat (I've never used it that way). The only thing, it's kind of a PITA to store as they don't fold (flat) like an accordian, they prefer to be stored as tube, so you can't slip it into a seat back pocket.
I also have the side shields for my front windows and so far (~4yrs later) they can stay in place without the provided suction cups