The link is a site to answer lots of questions about your patent, ith regard to the cost of it, it says:
"There is no longer an initial fee for applying for a British patent. Generally, fees of �200 or more are incurred in obtaining the grant of a patent. In addition, the fees of a patent agent could be �1000 or more for a straightforward application. Such costs (which may be higher), plus the costs of translating the specification into the relevant language, will be incurred in every country in which you wish to obtain patent protection. Once the patent is granted renewal fees have to be paid to keep the patent (or patents) in force. The British Patent Office site will give advice and can send a free brochure."
http://www.bl.uk/collections/patents/faq.html