We've just come back from an Easyjet trip with our 5 month old - pushchair was carried free of charge over and above the luggage limit of 20 kg per person, but as far as I know, carseats and travel cots would be treated as regular luggage in your allowance. I can't help on the bottles thing I'm afraid.
take empty bottles nad buy cartons of milk when you go through security (you can phone boots airside at most airports and they will put some aside for you) last time we flew (july) we took pram and car seat, neither was classed as regular luggage and we were not charged for them.
we went with ds when he was 6 months. stroller and travel cot are free, not sure about the car seat sorry! and i bf so not sure about the bottles either. could you bring your bottle and then buy the formula cartons at the airport boots? enjoy your trip