You can't go wrong buying from Bill McGrath. It might be worth asking if they'll store them for you too. They aren't far away from sunny Barnet. You will be shocked at how much of your valuable garage space a set of tyres take up.
Now down to tyres: I put Nokian GR W2s (225/55/17s) on my last car, Citroën C6. Nokian is a Finnish company which knows a thing or too about snow, they have been around for years you might have heard of them, one of their subsidiaries started making some mobile phones. The tyres were amazing and whether it was fresh snow, compacted snow, sheet ice, melting slush or a combination thereof they just took it in their stride and the car felt as planted as using ordinary tyres on a rainy day.
I see the GR W2 has been superseded with this
http://www.nokiantyres.com/tyre?id=21399126&group=1.01&name=Nokian WR A3
I bought them from mytyres.co.uk
You could order them from mytyres.co.uk and have them delivered to McGraths for mounting and balancing. You should also check McGrath's price. I bought my QP's P-Zeros from them because they matched mytyres.co.uk price for me.