You consented to give the interview, you accepted the money - you can't renege.
A contract doesn't have to be in writing to be legally binding - you did the task in return for money knowing from the outset the interview was to be broadcast.
There is all the elements of a legally binding contract in place - if you rtry to stop the broadcast they could, theoretically, sue you for breach of contract and ask for the �50 back plus all the expenses relating to doing the interview and court costs.