Be eligible to work in the UK
To take on a traineeship, you must be able to legally work in the UK. If you are not a UK citizen, you will need to check which visa will allow you to do this, as there are a few different ones. Information on each visa type, including how long it allows you to work for, is here. Keep in mind that traineeships last a minimum of two weeks, but can be for as long as six months.