Just following up - We flew easyJet and it went mostly fine. As others have said, you have to pay very close attention to your luggage & carry-on allowance, but we had a scale in Inverness so we were fine. (There were three of us, and with the kid & the car seat we couldn't have managed more than three carry-ons anyway, so no issues there...)

We did pay for speedy boarding, but it wound up being a waste of money. There was a guy in a wheelchair in the front of the line, and a bunch of 'senior' speedy-people in front of us. The wheelchair guy was sent up the front stairs, so we went up the rear stairs behind the old folks. By the time we got on board via the rear stairs behind all the older slowpokes the wheelchair guy was on board and all the non-speedy people were sprinting on via the front stairs. We got the last "row of three." I think the solution to this problem is to not board anyone until the speedy people are at least on board, but c'est la vie. Gatwick was also a bit of a nuisance. We held back to deplane with the little one so by the time we were off the flight and down the stairs we were the last one on the tarmac-bus at Gatwick and our little one was pretty squished. Once in the terminal you also have to walk a long way at Gatwick. Not sure if this is an easyJet issue, or simply the nature of Gatwick. Fare was good though