A UK patent lasts for 20 years from the date of filing the patent application (s 25).
However in order to last for the full 20 years, the patent must not be revoked and the renewal fees must be paid. Renewal fees are due every year after the first five years from the filing date. The purpose of the renewal fees (besides to make money) is in order to encourage patent owners who have patents that are of little use to them not to renew them (the fees also rise as times goes on).
If the patent is for a medicinal or plant-protection invention then a supplementary protection certificate is available which enables protection for: 5 years from the expiry date; or 15 years from the product’s first marketing authorisation in the EU.