Insurance is provided for all u3as by the u3a National body – The Third Age Trust. A questions and answers document explaining the details can be found here.
Please note, this document was downloaded from the National u3a site on 9.9.22 and although frequent changes are unlikely, you are advised to visit their site https://www.u3a.org.uk/index.php, to obtain the latest version. (You will need to create an account and login to obtain information from the u3a website, but there is a lot available, covering a wide range of topics. A search for ‘Insurance’ found this document)