In Touch with Dynamics (GP) has a look at VAT changes coming to the UK and how to deal with them in Dynamics GP. The article up at All Change on 20% Tax Again January 2011.
Dealing with UK VAT changes coming in 2011 with Dynamics GP
Archived post from 2010. Dynamics GP and the links it points to have changed since; check current documentation before relying on version-specific steps.