CARN (County Archive Research Network) Reader's Tickets are accepted by Record Offices that are part of the network, and are used as identification. The card is valid for 4 years and is available free of charge.
You do, however, have to provide proof of
identity using an official document that states your name and address and
provide an item that shows proof of your signature.
You may also be asked to provide 2 passport sized photographs.
Please note The National Archives does not accept the CARN card as you have to obtain their card the first time you use the Archives and this again requires you to take proof of identity along with you.