United Kingdom citizens do not need a passport to travel internally although it is a good form of proof of identity to take with you.
Yes, you are correct ... sort of. Practically every airline WILL require you to have some form on GOOD (photo) identity, even for a flight that remains within the UK. Typically a passport IS used (though not a legal requirement for adults), but other forms of ID are accepted too .. this varies by airline, so whoever you plan to fly with can give specific details.
There MAY be further issues, specifically with a flight to Belfast, as from here (IF someone had mischeif in mind), a person can cross unchallenged into Eire, and from there on wards internationally.
The issue of travel with a baby / small child is more complicated (this is from experience of doing so, within the UK borders and internationally, with our little boy, who recently had his 2nd birthday.
International flight DOES need a separate passport for the baby / child.
With internal flights, such as England to Belfast (or anywhere else within the UK), you WILL be challenged to show that the child is yours, and you are travelling legally with them ... particularly so if you travel as a single parent.
The simple fact is that for an adult to be travelling alone with an infant MAY be attempted for less than appropriate reasons: a recent separation between parents etc can lead to one of them fleeing with the child, which would normally be restricted. Abduction and worse also comes to mind, which is why the airlines can appear to be over cautious.
The simplest option would be to be able to show a passport for the baby (in the same surname as yourself), or at the very least, carry an original copy of the birth / registratioin certificate, which will show details of not only the infant, but also, usually, both parents as well.
Again, the airline with which you travel can advise on their specific needs and regulations.
A word of warning :-)
ANY fuss or concerns over identity at identification stage WILL result in the attentions of airline / airport security, and much deeper scrutiny!
Although it CAN be a bit of a nuisance, I'm fully in support of the measures as a parent, particularly considering why these measures are in place.
Yes, a baby needs a passport for international travel.
Yes, a baby needs a passport to travel internationally.
Yes, a baby needs a passport to travel to Canada.
Yes, a baby needs a passport to travel to Mexico.
Yes, a baby requires a passport to travel to Mexico.
No, a baby cannot travel without a passport. All travelers, regardless of age, need a valid passport to travel internationally.
Yes, a baby can travel domestically within the United States without a passport.
No, a baby cannot travel to Canada without a passport. All travelers, regardless of age, need a valid passport to enter Canada.
No, a baby cannot travel to Mexico without a passport. All travelers, regardless of age, are required to have a valid passport when entering Mexico.
Yes, your baby will need a passport to travel to Canada. It is a requirement for all travelers, regardless of age, to have a valid passport when entering Canada.
No, babies need a passport to travel internationally, regardless of their age.
No, a baby cannot travel internationally without a passport. All travelers, regardless of age, need a valid passport to enter another country.