I think, having read up on this, they are legal as long as they are not going to cause damage to the road surface, i.e. there is a significant covering of snow and ice.
The MOT Inspection Manual makes reference to studded tyres, so they must be legal in some shape or form, in fact every semi-authorititve page I have seen on the web suggests they are legal, as long as they don't cause damage.
I really can't see the justification for the month or so of snowy weather get a year, winter tyres would be a much better bet.