While in theory many international companies can move few will actually do so for the following reasons:
1) They lease or own office space which will be have to be subleased otherwise they will have to take heavy loss on them. The Saudis bought mansions in the UK which are vacant although there maybe some squatters living there.
2) Many of these firms are in the financial sector and if they move to the European mainland they may not be able to find new employees with the same level of experience. Also they will have to provided severance packages for these laid off employees. Also with the loss of so many jobs the Bank of England may decide to help these financial experts compete against their former employer luring away clients. Likewise in manufacturing sector, they will only get a penny on the dollar for it and if the government want to extend the former employees credit they can buy it and run it competing against their former employer.
There are risk involved but I don’t think it is as bleak as portrayed.