http://www.nationwide.co.uk/hpi/historical/Mar_2008.pdf
What is interesting is that nationwide believes that they will not expernence anything similar to the US, an interesting comment. However, trends always overshoot to the downside as much as they do to the upside; their own charts clearly illustrate this.
Based on historical Real Estate bubbles (IMF) the UK could see a 30% drop in prices, over a period of 5 years.