You can close your website for registration and invite people with the e-mail addresses @(uni).ac.uk to join your website
this will be the easiest way, of course if you know their e-mails. Other than that, it is a custom code modification as there's no such functionality in the default software.
You will need to do that on your own or hire a programmer to do that for you. You can do that here: http://www.oxwall.org/market/specialists
Also you can make a post in the custom code modification section of the forum, probably some community member will assist you with that.