Changing the registrant on a Nominet (.uk) domain has 2 main steps:
1. Nominet domains have an 'administrative' email address linked with them, which allows domain name owners to manage their domains via Nominet's control panel. By default, we set this to be one of our domain admin addresses. So firstly, you have to update the administrative email address linked with the domain.
This is done via the MyFreeola account in which the domain is managed. Select View/edit domain details from the left page navigation then Hosting and Settings > Change Address (not change registrant).
On this page you'll see a tick box worded "Use your MyFreeola login address as the administrative contact for this domain". Tick this box and confirm. Now, the admin email address on the domain is your MyFreeola address.
2. Call Nominet and they'll ask you to confirm the administrative email address and go from there.
Also DL mentioned that leaving the domain name to lapse so it can be re-registered could be an option. It could, but it's worth pointing out that most domain name types don't become available for re-registration until a point between 70 and 90 days past the expiry.
However if neither Freeola or Nominet won't take the time to assist you, then just let the domain name registration lapse. The domain name in question will then become freely available for anyone to register and thus, not in your name anymore and so no re-registration fee will be required. If your ex wishes to take it onboard...he can do when the/your registration expires.
Sorry about your separation
Maybe Hmmm will be along later and advise you otherwise