Right to Work
When you take on any employee, as an employer you must ensure that the potential employee has the right to work in the UK.
There are many different types of Visa and passport and lots of changes will be happening in the near future, so it is important that you check that your potential employee is allowed to work in the country and in the type of role that are offering.
You must also not to discriminate by not asking one employee for proof but then asking another.
The best thing to do is to complete the governments right to work questionnaire for every employee that you take on, this will tell you whether they have the right to work, what identity documents you should take copies of to prove this and whether you need to check them again at a later date.
The link to complete this is https://www.gov.uk/legal-right-work-uk.