Just in case anyone else has domains that have automatically renewed on Nominets new (unnecessary) price increase, you can un-renew them through Web Domain Manager. Apologies if everyone already knew this but I didn't until today. Login to Your Nominet control panel > Web Domain Manager > Click the domain in question > At the top right hand side of the page you will see a button to 'Unrenew' You can only do this if they haven't been invoiced. Once Nominet raise the invoice the facility is not available.
Isn't there a limit to the percentage of domains you can do this with? Long time since I read the tag terms, might have changed.
There is a limit of 5 or 5% (which ever is greater) to the number of domains you can delete prior to the invoice. Not sure it is the same for un-renewals.