LYKA
I'd take issue with your first point. I don't know how close you consider 'adjacent' airfields, but if all the airfields in (say) the Midlands were simultaneously affected by snow to the point that braking action was seriously degraded, there's no debate about the safe option. They close to snowclear. All the airlines have to carry their holding / diversion fuel. The crew will have access to the weather forecasts or actuals, which will let them determine if they need to either nominate a diversion alternate further away from destination, or carry extra holding fuel if they're likely to be delayed upon arrival. It would have to be a sudden, severe snowfall covering a huge, huge area to close destination and all suitable alternatives.
The alternative risk, of sliding the aircraft (probably with still thousands of kilos of fuel on board) off the runway upon landing, is of another magnitude.
Of course, if a crew decides that things are so bad that the safest option is to land on a heavily contaminated runway, that would be permitted, but it really would have to be pretty serious.
As for the thunderstorm bit, there's guidance in a recent-ish AIC to crews regarding operation in thunderstorms. Generally the UK doesn't get storms as intense as, say, the US, so we get less problems with microburst, tennis-ball sized hailstones etc etc. Weather radar lets crews decide where the worst bits are and they'll either delay departure or request to hold off an approach if necessary.