An isp has to become a Nominet tagholder in order to manage .uk domain names. When an ISP becomes a tagholder they have to sign a tagholders agreement. This agreement states that they must do a tag change without any unreasonable delay providing they have the authority of the registered owner of the domain.
If a tagholder is refusing to change the tag on your domain please contact Nominet and make them aware.

You can get the tag changed without the approval of your ISP, to find out more about this process please go to the nominet website: . There is a charge made by Nominet of £15+vat to do this. You will have to provide proof of identification.