This is called cybersquatting - when you own a trademark and the other registrant has registered the domain in bad faith. There are means for resolving these matters. I have not been able to find any laws specific to the United Kingdom, however there are other laws and agreements this falls under.
However, the respondent (the person owning the domain in dispute) must generally intend to profit from, and have registered the domain in bad faith. They must also have no legitimate interest in the domain name. The applicant must also show that it is similar to their trademark.
You may be able to submit a dispute to ICANN in accordance with their Uniform Domain-Name Dispute-Resolution Policy. The decisions of such arbitration is binding on the parties, and may include cancellation or transfer of the domain, or refusal of the claim.
If you are able to bring a claim in the United States, the Anticybersquatting Consumer Protection Act provides similar protections, and may additionally award damages.
Additionally, the World Intellectual Property Organization provides for arbitration, which allows trademark holders to attempt to claim a squatted site. However, these decisions are not binding on the parties.