You can take everything.
When I went to London with Maggie, a couple of months ago, I took a bottle of her water, and then her bottles, and the formula, all in the nappy sack. The water just goes through first., but the formula, they dont even look at!
We only have one brand of ready made formula here, in supermarkets, and its not the brand she drinks, so it never even crossed my mind to take some!
When I went to London, I took my travel system, with the car seat, as I was gonna go on a friends car. And honestly, I thought it was great! Sure, my pram is big, but it also means I could carry all my luggage on the pram! My carry on went in the basket, Maggies bag went across the from, plus the nappy sack! It was GREAT!
In terms of sleep regression, Maggie didnt have one. Maybe a couple of worse nights, but nothing major. On the whole, she sleeps through the night every night.