All UK boilers are given a SEDBUK (Seasonal Efficiency of Domestic Boilers in the UK) rating. This tells you how efficient they are.
There are currently two types of SEDBUK rating. SEDBUK 2005 rates boilers from A-G, with A-rated boilers being over 90% efficient. SEDBUK 2009 is a newer system, and rates boiler efficiency in percentage.
All new boilers should be rated using SEDBUK 2009. But if you have an older model then it might have been tested under the 2005 system.