Once a Patent Application is filed in the UK it takes approximately 18 months for the UK Intellectual Property Office to conduct the initial examination.
Once the examination is complete and if the application is to proceed to the next stage it will be published on a public register. The purpose of this publication (as with a trade mark application) is to allow any third parties to object to the registration/ grant of the patent.
If no objections are raised the patent will likely be granted approximately 6 months after publication. Therefore on average a patent application in the UK takes 2 years. This 2 years forms part of the 20 year validity of the patent. This is also true if complications arise before the grant of the patent and the grant takes longer to be achieved.