Whether already here, or looking to relocate, companies plan several years ahead, looking at political stability, the tax regime, infrastucture and if the country has the skilled workers it needs.
“EY: ”Cutting UK tax draws in more multinationals”
www.telegraph.co.uk/finance/newsbysector/banksandfinance/10472446/EY-Cutting-UK-tax-draws-in-more-multinationals.html
“The accountancy firm highlights that the UK's competitive tax regime has led to a rise in overseas companies interested in domiciling in the UK”
Also as English Law is seen as the best for business legal, financial and other contracts - that is another attraction.
If the UK remains a business friendly environment, there is no reason for them to leave en mass, but be warned, once the decision is made for them to leave, it can take several years to get them back, if ever.
As an example, Manufacturing, around 22% of our economy fell from 22% in 1997 to 12% in 2010, with 1 million jobs lost before the crash.
Five years later, the last figures I saw was manufacturing was accounting for around 15% of our economy.