You need photo id. Different airlines have different rules as to what they will accept - a passport is always acceptable but other documents (eg photo driving licence) MAY be OK.
bmibaby accepts all forms of photographic identification, however passengers must ensure that all personal identification is of a good standard. If identification appears to be fraudulent, passengers may be refused for travel.
blondmam, I agree with dzug. We flew from Bristol to the Isle of man about 3 years ago and at the last minute we decided to take our passports, because someone told us it was advisable. We needed them, but there was nothing on the documents we'd received to tell us this.. So as dzug says, check with your airline.