Mildly? Coming here to work won’t make sense on several levels, so loads of low wages jobs won’t be done. It’ll be a disaster for farmers and the food industry generally. Probably be the same for catering. I’m not sure about high skills stuff, people are more willing to deal with the visa issues there.
The main upside is that a sterling collapse inflates away the national debt. When that’s your upside you know you’re fucked.