A 'Citizen Card' (of the type used by young people to prove that they're old enough to buy age-restricted items) is not valid in place of a passport or national identity card. (However, some airlines might accept such a card as proof of identity, for security purposes only, for travel within the UK or for travel between the UK and Ireland).
Travel to a non-EU country always requires a passport.
The 'six month validity' rule is largely a myth. Only a few countries operate such a policy. (Even the USA, which has some of the strictest entry requirements in the world, exempts UK passport holders from their normal 6-month validity requirement). All EU passports (including those of the UK) are valid, throughout the whole of the EU, right up until the last second of their stated validity.
You can check on the entry requirements of any country (including whether you need a visa or whether you need a minimum period of validity on your passport), using the Foreign & Commonwealth Office website:
http://www.fco.gov.uk...el-advice-by-country/
Chris