Please see the UK Home Office website to find out if you are required to apply for a visa before travelling to the UK and for more information about Visitor Visas.
You should have received/will receive a letter from your Hub with your mark statement after you had/have successfully completed your course that states you will receive your certificate at the graduation ceremony held on a certain date. If you take that letter with you to show when you apply for your visa along with the Graduation registration confirmation email this should be sufficient evidence to show.
If you are planning on inviting family from overseas please note that the University is not permitted to issue letters inviting family members to the UK or even provide a template letter. For further information please consult the Student Support Services help guide or alternatively please email ISAT@UEA.ac.uk