When UK voted to exit EU, it meant that UK’s leadership would work towards dismantling various economic & social treaties with the EU.
This caused uncertainty in the British market. Thus, investors lost faith in Britain’s economy leading to a fall in GBP (Great British Pound) & FTSE (British Market Index). The GBP was already trading at a 31 yr low, thus the already cautious investors were given a rude shock
UK has a large immigrant workforce. Many of them reside in the UK either directly or indirectly due to UK’s membership in the EU. Their loss could lead to stagnation in the UK economy.
This scared investors further & hence, they pulled out in order to wait for the political situation to stabilize.