Beats me. It does seem an extremely low conviction rate, though. Maybe British prosecutors are bad at handling such cases, or the court system doesn't take them seriously. Check out this URL, comparing conviction rates in the US and the UK. graph on the upper right.
Woops--forgot the URL:
http://www.ojp.usdoj.gov/bjs/pub/html/cjusew96/cpp.htm
US Dept of Justice, Bureau of stats.