I decided to follow in the footsteps of el mambrino, and purchase glasses online. Unfortunately, Zenni Optical, who came highly recommended by the aforementioned mambrino, don't seem to deliver to the UK.
Instead, I decided to go with Glasses Direct, mostly because I had seen a pair I liked in a catalogue at Joanne's parents' house a few months ago. They came in the post today. Joanne said that she thought that they were "too hip" for me, but I really like them.
Finally, I'd like to thank Glassy Eyes for all of the suberb work he's done in finding great deals on-line and for providing simple technique of calculating pupillary distance (PD).