Just looked up the current photo ID requirements for bmi as i'll be making my first flight with 3-month-old DS in a couple of weeks. Searched here first but didn't immediately find the info so i'm posting it for other Mumsnetters.
The upshot is: bmi don't require infants to have their own photo ID for domestic flights.
"Acceptable forms of photographic ID
Travel solely between the Republic of Ireland and the United Kingdom.
Acceptable forms of photographicID include:
?Valid passport
?Valid photographic driver's licence
?Valid photographic EU/EEA or Swiss national identity card
?Valid armed forces identity card
?Valid police warrant card/badge
?Valid airport employees security identity pass
?Valid business photographic ID (Dublin only)
Citizens of Ireland and the UK under the age of 16, do not need a photo ID, if travelling with their parent/guardian.
Citizens of countries other than the Republic of Ireland and the UK must produce a valid passport and visa where applicable, or national identity card (EEA & Swiss Citizens), for travel between the Republic of Ireland and the United Kingdom."