While many international companies may say they will move from England, how many will actually do it. Will they break their lease on office space and fire their British employees and hire new ones. The cost for these companies will be enormous. In this day and age when many employees are telecommuting ie. working from home, it doesn’t matter where they are located.
As far as starting a trade war over this , the UK has the British Commonwealth (former colonies) to rely on as trading partners besides the US. Similarly a trade war will result in the UK withdrawing from NATO encouraging Russia to expend it’s area of influence.