Ask the British "Home Office" (interior ministry), who publish an email address for public enquiries. The Independent newspaper refers to a government definition of "extremism", which the Home Office will be able to tell you where to find if you cannot find it. There is not much point in the government's referring to "fundamental British values" if they don't define the term. My guess is that they do define it somewhere.
"Extremism" can sometimes be a silly term, insofar as one can ask "extreme what, exactly?", and one can suppose that whatever the "what" might be, it may well be considered acceptable by the opponents of the said "extremism" if it manifests in a "moderate" form.