Plus....if you do use the iplayer without a current licence then under current legislation you can be fined or imprisoned. I am sure mind, that the "authorities" will only pursue certain members of the public with vigour, and implement the full weight of the law.
You have to register online to watch a few programmes (Just your email address and a password)) - probably just the nature of the content, but like the others have said it is completely free. I use it frequently and have never been asked for payment or any card details.
Regardless of being asked for payment or card details up front, you still need a current licence. Since 2016 this also includes using the iplayer to watch catch up tv or prior recordings.
I agree with Fitzer.....the OP should double-check his broadband allowance. Although uncapped (ie unlimited) is now the norm, he may still be on a capped service....in which case, using catch-up services a lot may well exceed his limit and cost him dear.