Visa for taking up residence in France or Switzerland only for UK citizens:

If you are coming for less than 90 days per period of 180 days:
No visa or work permit required for France or Switzerland but a convention d’accueil will be required (equivalent to a work permit). The teamleader of the person coming to CERN can request a convention d'accueil via the EDH Invitation Letter Request. For help reguarding this document please see https://usersoffice.web.cern.ch/help-page-invitation-letter-obtain-visa-or-convention-daccueil . Please note that the Teamleader's name will appear on invitation documents produced via EDH so they are responsible for the person concerned while they are using the documents. If the person leaves the home institute before the expiration date of the invitation documents then the Teamleader must tell the Users Office to cancel the person's User registration which will render any invitation documents associated with the registration invalid. Please note that the invitation documents provided are official documents so must only be used for coming to work at CERN. 

If you are coming for more than 90 days
Living in Switzerland
No visa is required and related Swiss/French cards will be requested by the Users’ Office upon arrival.

Living in France
VISA D is required.
The teamleader of the person coming to CERN can request the required invitation documents via the EDH Invitation Letter Request. The person can then use the documents to apply for a visa via their local French embassy. Related Swiss/French cards will be requested by the Users’ Office upon arrival.

You  can find all information on the CERN dedicated web page.

Please note that it is the Users responsibility to ensure that they have the correct documents and follow the required procedures for entering Switzerland/France.