We have used our Britax Eclipse on Easyjet flights with no problems, it fit on easily with the lap belt. It is one of the narrowist car seats. The only bother was we had to pay for another seat and lug it about everywhere
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. It was worth it in the end though as I didn't have to have my wriggly 18 month year old on my lap for take off and landing ( using a car seat on board is the only way around it) Just noticed your little one is over 2 anyway so it wouldn't matter as much
Check with the airline first and they will tell you their requirement, e.g. must have five point harness.
Katy