Why do airports in the UK have so few runways?
There are perhaps 101 non aviation-related reasons why the UK does not have larger airports, such as space consideration (we're only a small island!), politics (NIMBY!), civil engineering (Airports are commonly near urban centres and are often surrounded).

However, the most aviation-related related reason I can think is that we just simply rarely need cross runways. If you look at most large airports in the UK their runway(s) are east-west(ish). The reason for this is over 70% of the prevailing wind direction in the UK is West or SouthWest. 

In addition, we rarely have extremes of wind - those exceeding the crosswind capabilities of common commercial airliners. Heavy snow is even rarer. I can probably count the number of times a runway at a large airport has been closed due to any of the reasons you mention in the last 20 years on two hands.

Bottom line; it's just not necessary. There is no socio/political/weather climate to warrant it.