For those already working here, it is not a big deal. For those who are yet to come there will be several obstacles for certain. It is still soon to know how Brexit will affect future visitors. What I know for sure is that they want to find a good compromise with EU cause if it will be hard for those outside the UK to come here, same treatment would be reserved for British citizens who want to travel or live in EU contries :)
For those instead looking for a job in the UK after Brexit effectively takes place, well yeah it will be hard. Probably they will need to be pre-requisites. They might need to find an employer first. Which is obviously more difficult.