molgrips,
how enforceable a road closure is will depend on different factors (e.g. failing to follow the instructions of a constable if he is actually present is likely to get you in bother). Proper snow gates usually have a “no motorised vehicles” sign on them which would be enforceable; red rectangular signs may not be if there is no accompanying road closure order. But see this: http://www.guardian.co.uk/commentisfree/libertycentral/2010/jan/11/police-road-block-rights
If you skid off the road or hit someone else coming the otherway I’d expect your insurers to raise a few eyebrows.
If there are jacknifed lorries then (1) you won’t get through even with a 4×4 if they are completely blocking the road (2) there may be people working to try and clear the obstruction who assume they are relatively safe (blind bends / summits, poor vis etc?) because the road is closed.
One of the problems when people get stuck is they abandon their cars on the road, then when someone comes to clear it (1) there is an obstruction (2) there is a vehicle in a stupid place delaying the road opening or presenting a hazard to drivers. You might be able to get your car somewhere more sensible – but my experience is when the roads are impassable the laybys are even worse (as a plough has dumped a load of snow in it). If the snow is drifting your stop may be more abrupt than you think. I’ve not idea how bad it is where you are – but if its a main route and they have closed it for more than a few hours whilst waiting for a plough then its probably a bigger obstruction than you think.