hawkwalks link gave a good explanation but I did have to laugh at the opening statement that said
�Visitors are informed that in the United Kingdom traffic drives on the left-hand side of the road. In the interests of safety, you are advised to practice this in your country of origin for a week or two before driving in the UK.�
Errrr yes, practice driving on the left in your own country for a couple of weeks, that might be good for our road safety, not so good for the road safety for the country your practicing in I should imagine!!
:o)