Each Domain Name registration agent is identified by a unique IPSTAG assigned by the UK Governing body for UK Domain Names, Nominet. If you want to move a UK domain name to another Internet Service Provider, you will need to know the Tag Name of your new ISP.
The majority of requests to move domain names from one ISP to another are carried out by the registration agents, also known as "tag holders", themselves. If you want to move a domain name to a new ISP, you should start by contacting the current tag holder as they will most likely have a process for this. They may charge you an Administration fee. See the FREE and FEES sections here.
If the current tag holder is unable or unwilling to help, you can ask Nominet to intervene.
How do I identify the current tag holder?
You can check the current IPSTAG for your domain name by doing a
WHOIS check on the domain.
Look for this:
Registrant's agent:
Yoursite Etc.. [Tag = YOURSITE] <<
URL:
http://www.yoursite.com
Look for the "Tag =" and that is theIPS TAG. So then the IPS TAG would be YOURSITE.
Source: "http://www.transferdomains.co.uk" blog