Ryanair makes no money on its flights

It makes all its money on baggage charges, check-in desk use fees etc

Staff earn commission on the amount of extra charges they manage to extort at check-in

No flexibility

Even worse, if they demand extra money you cannot pay at check-in in London. You are thrown out of the queue and sent to join another long queue to pay the extras, at which point you return to the queue you were originally in (unless you get a nice agent who'll let you push in, as I did last time it happened to me).

(PS. And 10kg is almost nothing, unless you intend to pack your own pillows)