I forgot to add, PSU's made by companies that care about efficiency will have an 80plus logo which means they have been tested and certified to have at least 80% efficiency at 20 50 and 100% loads.
However please note that it is not clear whether they do these tests for 220/230v as well as 110v so although the list of PSU's is relevant the figures might not be accurate for use in the UK.
See the current list here http://80plus.org/manu/psu/psu_join.aspx