British Airways Downgrading PAX

I tried to check in online on 2 February 10:00 and pre-book and check in - it let me choose the amount of bags I had etc.. and when I came to final stage of check in it stated I can only get boarding pass online - <br> I though the system was down and left it there. When I got to the airport at 14:30 I was told I was downgraded from a fully paid premium economy ticket to economy due to the flight being overbooked and they said I didn't check in online - and I know I definitely did!<br> I was treated very rudely and no one tried to even check or help me. I feel with the Consumer Protection act if you pay for a product or service in full and don't get it you need to be compensated, Airlines are not allowed to overbook or oversell a flight if the remuneration to the client has not been offered in return - I was treated unfair and without any compensation. I want half the portion I paid on premium economy back seeing as I didn't fly premium. I will be travelling this route this year at least 6 times and will rethink whom I fly with as your service and manner you treat your clients are very poor. I also feel the airline need to compensate me for a certain expectation I had and did not get.