We have registered a lot of .uks for people where the .co.uk is not with us, mainly in circumstances where they're with a registrar that wasn't ready or set up to handle the .uk registrations. Nominet will allow it as long as all of the registrant details including the email address match. Nominet will then send an email to the registrant email, asking them to confirm that they have indeed registered the .uk version of their name.