So glad to see this message, as we too have a Maxi Cosi (aren't they great?!), and need to take it on a flight a week tomorrow.
We are flying with Air 2000 to Turkey. I haven't been able to get through to anyone by phone (constantly engaged or on hold), but checking on their website, they say that it is OK to take a car seat for use on the plane (ie to strap your child into for the duration of the flight), providing it's no more than so many inches wide (I think it's 16, which doesn't sound much). You could ask Easyjet the dimensions of the seat, and then measure yours).
If you turn up with it and it's too wide to use on the seat itself, then you'll just have to use one of those lap belts. If you haven't used these before - they have a loop that you thread the regular belt through.
I have flown Easyjet with my daughter and there was certainly no problem about taking one of the Rockatot style baby car seats. In fact they were really helpful. We just checked it in with the luggage (make sure it's well padded though, and wrapped up in a binliner, to prevent damage. It just came out the other end with the rest of the luggage.
Hope this helps. If anyone has got any experience with Air 2000 relating to this question, please let me know.