"I expect if the UK left that many multinationals would move their European headquarters out of England / Wales and into Ireland or Scotland. Oh they might leave a rump staff behind to deal with the UK but it would be cold comfort for those whose jobs were transferred elsewhere."
My employer has already shifted focus and new kit and jobs are mostly now going to the continent and not here. That's having a knock-on effect on the smaller businesses that we buy stuff from because that business is now going to the continent too. Power, air-con, lifts, fire suppression, comms, metalwork, cabling, security - the risk of Brexit is costing British businesses that supply that stuff money and jobs right now.
If you were going to open a datacentre this year, where would you put it given the risk of Brexit? The EU is a much bigger market than the UK and putting it here risks a stranded asset that can't be used by EU customers if data protection rules play out badly following a Brexit.
Even if some kind of favourable trade deal could be workd out post-Brexit, I can de-risk my investment completely by shifting that spend away from the UK and to France or Belgium or Germany. Why do you think the Euro is continuing to gain in strength against the pound? Follow the money.