If anyone wants to know about a seat that works with Easyjet and KLM, I used it on these airlines, it was a TWE (Two way Elite).
The experience was ok, there are + and - points about taking it onboard.
The TWE fits well between the seats on both Easyjet and KLM, there is no width problem as it is quite narrow at the base. The only thing is the seatbelt on Easyjet is only just long enough. You need to push the seat into the back and have it sit upright, then you can just manage to click the seatbelt buckles together. Once that is done, the rest is easy. (You are not allowed to use a seatbelt extender for this, according to crew, so you have to fiddle a bit).
Our son was much more comfortable in it too, as he sits much higher and can reach the table. Second, it is his own seat that he knows well. Much safer too.
What I do is I take the two tetherings straps of the TWE and attach them to the seat through the back holes, then you can carry the TWE on top of a handluggage suitcase with wheels by looping it around the handles. That makes it much easier to walk around the airport.
Lastly, make sure you read the easyjet rules, or better print the bit about car seats and take it with you. The check-in people will sometimes freak out as the seat looks huge. But when you speak to the manager and point out the width is fine, and it is 5 point harness etc. they have always accepted it. The best thing about taking it on the plane is perhaps that you do not have to wrap and unwrap it because you don't have to check it in! That took me 20min sometimes.