I believe it is possible to buy a fleece cover for a plastic greenhouse but the biggest problem with the plastic greenhouses is that they have a tendency to get blown over in winter. Which is quite heartbreaking.
Mine never proved to be completely frost free so it was ideal for small hardy perennials/ annuals that needed over wintering but not suitable for propagating pelargoniums/fuchsias etc.
Tracie, Kate lives a lot further south than you or me- am in Manchester. So we have colder & longer winters than she does.
Ordinary, low rise coldframes- wooden, plastic, aluminium framed- can be successfully insulated with sheets of polystyrene, such as comes with packaging, around the sides, with extra fleece sheets on top of any plants inside.
I would only tend to use a plastic greenhouse for over-wintering small hardy plants, usually grown from cuttings taken earlier in the year. By well wrapping the actual pots of bigger plants, plus using a couple of fleece sheets over them you should be able to keep some things. Dahlias, pelargoniums & other non-hardy plants I wouldnt personally risk.
I'd certainly search out the actual correct sized fleece cover for your greenhouse & get one, plus extra fleece & be prepared to really secure the whole to a wall/fence as securely as possible. J.