5m is a lovely age for the first flight - S was 6m for hers and it was pretty easy.
Formula is fine in your hold baggage, just double-bag it in case it leaks powder everywhere.
We did varying combos of steribottle, powder/water and readymade formula, which might look something like this:
Formula made up in a Steribottle to give S a final feed before going through security (bottle then thrown away).
Sterilised bottles in hand luggage, either
a) with boiled water and formula powder, which we had to taste (tastes of potatoes, yuk) or
b) empty, then we bought readymade cartons from the Boots the other side of security (this is Gatwick and Stansted, I believe you can call ahead and reserve cartons in other airports if they don't have big shops?)
You'll need enough bottles/formula for 1 before security, 1 on the flight (perfect for take off and landing sucking), plus at least one the other end before you get your steriliser etc sorted. Again steribottles can be fab for that last feed (remember to save some for doing the whole thing again coming back!). I believe if your child is on a formula for medical reasons it counts as medicines, and most airports will give guidance about those, but you will want to check.
In your hand luggage, mini Calpol sachets are fab, as are alcohol wash bottles (subject to size limits). Take a changing mat and you may be able to change Dyl on the seat of the plane (loos on planes are grim).
Car seats are generally acceptable as extra to your baggage allowance, but check with airline. If it's part of your buggy/travel system you can usually leave it at the entrance to the plane with the buggy. If not they will let you check it in.
S has flown over a dozen times in 3 years and gets better every time - and it gets easier too!
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