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Nominet try and contact the registrant for renewal and if they have no contact within 7 days of the renewal date, they send an invoice to the registrant via email and post.
If the fee is not received in 30 days (from date of invoice), the domain is suspended for 60 days.
After this, the domain name will be cancelled and available for re-registration.
From: Nominet domain name renewal page [nominet.org.uk]
IMO, it gives way too much leeway to the domain registrant. They know for 2 years when the domain needs to be renew and not having it done on or before the renewal date is just sloppy.
Just my 2c tho! :)