In the first instance you are.not supposed to register the .uk of the name, since you knew you were going to sell it and as you have registered it now, it is better to send it to them to avoid problem for your self in the later days.
You may ask them for token and if they refused, let them know that it is their legal right to own the two domains but they may not make use of it.
If you already hold a domain within the .uk domain family that is eligible, we have automatically reserved the .uk domain for free, until 10 June 2019 as long as your unique domain remains registered.
Nominet is not 100% clear, however, the way it was meant to work was the .uk was automatically allocated to the owner of the .co.uk to avoid squatting. Every owner of the .co.uk has the option to register and pay, for the .uk before five years elapses, from 10 June 2014.From what I remember, the oldest subdomain registration takes prededence. If someone has a .org.uk or a .net.uk (etc) that predates the .co.uk, then that may take precedence. The .uk domain was reserved rather than allocated as there are another few steps to go through before the .uk registration is accepted.
If a .co.uk version of this domain name was held on our registry at the time of the cut off date of 28th October 2013, the registrant of this domain name has the right of first refusal to the .uk equivalent. Providing the domain name remains on our register, the right of registration will be in place until 10th June 2019, during which time only the registrant of the domain name would be able to register the .uk equivalent.
If the .co.uk domain name was not registered at the time of the launch date, the right of registration would go to another variant. The .org.uk would get the right over .me.uk and similarly .me.uk would get the right over .ltd.uk.
If no registered domain name existed that granted the right of registration on the 28th October 2013, but the .co.uk domain was registered between the 28th October and the .uk launch date, the .co.uk domain name would then get the rights to the .uk.
If you have both the .co.uk and the .uk domain names and you sell one of them to someone else, you are able to keep the other one.