Annex ZB of BSEN 61439-3:2012 has a Deviation for the United Kingdom that states that distribution boards intended to be operated by ordinary persons with double pole isolation, normal protective devices and type tested for use with standard supply fuses should be able to withstand a specified test using a fault current of 16kA and not fall below certain levels of safety during and after the test e.g. not cause fire and not be significantly damaged or deformed.
Therefore all UK consumer units should be designed to withstand a 16kA fault current and so this is the maximum level you can specify on the EIC.