There are at least five other categories of British 'citizenship' or 'nationality', in addition to the full British citizenship which entitiles the holder to a full British passport and the associated EU (not EEC any more!) privileges.

UK Passport Service : Who is eligible?
These categories were defined under the British Nationality Act 1981, whose major effect was to restrict the rights of people from former colonies and commonwealth citizens to residency in the UK.

British High Commission- Canada: Passports: Who is eligible for a UK passport?

Home Office Immigration and Nationality Directorate: Applying for British Nationality