The British Library provides an online service for requesting UK PhD theses - EThOS (Electronic Theses Online Service).
You can search for these by author, subject or institution and you can limit your search to those theses that are available for immediate download. You will be required to register with EThOS to take full advantage of the service but this is a one-off registration which is there to help protect the copyright of the thesis.
Most universities are making their theses available free, however, a small number require payment for digitisation of the thesis. If you are asked to pay for the digitisation of the thesis or the thesis you require is not available through EThOS then please contact a member of library staff.
For full information go to http://ethos.bl.uk
For more useful sites go to Theses - useful links