The rules for cabin baggage, which are common to all airlines and all airports, within the EU, are here:
http://www.dft.gov.uk/stellent/groups/dft_tran ssec/documents/page/dft_transsec_613514.hcsp
You'll see that in general, the maximum amount of liquid in a single container must not exceed 100ml. However, there's an exemption for baby food (which obviously includes milk). There is no specific maximum, so any reasonable amount, for the duration of the flight, will be allowed. As you state, baby foods are subject to 'authentication'. (The exemption for baby foods doesn't leap out at you from that web page. It can be found under 'Medicines').
Liquid items, of any size, which are purchased 'airside' are permitted on board the aircraft. However, you should remember that outlets such as Boots, at airports, are primarily geared up to selling cosmetics, so you might not find anywhere selling baby foods.
Chris