The essential background
As I've been pointing out for some months now uncertainty is a pernicious killer of economies and thus market prices. It's as JM Keynes said: Business investment is the great variable component of GDP. Whether investment takes place is driven by the "animal spirits" of businessmen. When they face uncertainty - not just probability but genuine non-knowledge of the future - they won't invest. This is what tips an economy into recession, is what drives the entire business cycle.
The UK has had more than three years of Brexit uncertainty: whether it