"Where do companies make their money - that's where the HQ's are likely to be located."
Wrong. They are located where their (largely imaginary) contribution to the company's added-value attracts the lowest tax rate. But of course for the EU market, that is very likely to be somewhere in the EU. However, all this is old news. How is your MP going to vote in the Commons? If you want him/her to vote against Brexit, send an email. Now.