Basically, with some research, i have found, possibly the cheapest website for a Pay as You Go Sony Ericsson K800i.
Simbob2005 found a good website, but I think there is too much messing about with www.greatuniversal.co.uk
So, I found something better. Go to:
marshallward.co.uk and search for K800i. You should see the K800i here, connected to O2, for £199.99. Now there is a delivery charge, £3.50, I think, but all this saves you the messing about with Great Universal.
So, use the website I gave! And enjoy...