It would be a bit extreme and sad to have so many people leave.
In terms of impact, there would not really be much really for the people. They would once again be able to afford housing. Some jobs would go but the essentials for living would once again be affordable. You wont be able to afford as many luxuries but you will be able to afford security. Our economy has taken a very hard hit from mass immigration and hasn't been allowed to catch up. Stalling future immigration is more important towards those ends.
Globally the economy will tank as the UK realestate market collapses but that's all virtual anyway.