bkehoe
No problem, write them a letter outlining your loss of enjoyment on the flight due to their breach of the contract by failing to provide the seat you paid for.
Ask for a refund of the cost of the seat, pus £50 for loss of enjoyment (couldn't sleep due to non reclining seat etc)
Give it 7 days, if they don't reply or you aren't happy with the reply, give them a final warning and let them know (in temperate language) that the next stop witll be the small claims division of the county court, set a deadline of 10 days
If you get there, you will be able to have a good laugh when the administrator questions (a) why they were not able to manage their own policy by allocating seats to minors that comply with that policy (b) why they decided to breach their contract for the seat and to provide another seat with inferior comfort, when you had paid specifically for a seat with recline - if they argue they reserve the right to change seats, that will be a huge laugh given the lad offered to move and they forced you to and (c) rest assured that the cost of defending the action will be many time more than any compo you get.
Like other posters, I understand the reasons for this policy, however it does not excuse this type of jackbooted behaviour by cabin crew.
You were bullied, IMHO and bullying should be punished.