The CAA have obviously been worried enough about the subject in th past to have issued an Aeronautical Information Circular "Aeroplanes inbound to the UK with fuel reserves approaching minimum" - AIC 82/2003.
http://www.ais.org.uk/aes/pubs/aip/pdf/aic/4P058.PDF
You may need to be registered to view this.
The AIC is quite specific about the amount of fuel to be planned for. Sorry, it's in pdf form so can't easily cut and paste.