There is some material on the Valuation Office Agency website that explains the basis of the square meterage calculations. In general Net Internal Area is used and it explains what that means and what it includes and excludes (e.g. loos and cleaning cupboards are excluded) - I was looking at it only last week myself. As to the rate used per square metre, you will have to ask the VOA but the rate depends on the type of business you are.
http://www.voa.gov.uk/business_rates/index.htm