A simple question and we already have 8 different answers (with a range of about £10,000 difference) - Oh my God !
You have to be more specific in the question - as anyone of the answers above could be deemed as correct (depending on what spin you put on it)
Firstly (you asked for the UK average) - Most gov agencies don't include Northern Ireland in their figures (which reduces the figure a great deal) - The Northern Ireland Office for Stats has a seperate average yearly pay of approx £17,000.
(Secondly do you mean before or after tax, what age groups are you taking into account (the national stats website only includes over 18's I believe)
You also didn't state whether you wanted to include part-time workers, cash in hand workers or other types of employment. If you just want full-time employment you should get an answer of between £18,000 - £22,000 from what I remember (although most establishments will claim around £23,000)
The national stats website also includes bonus and overtime payments and is a very limited 1% sample.