Domain name and domain registration question

Can someone explain I'm a little confused I purchased loads of .co.uk domains and no issues, and then I purchased a .com and a .info and all of sudden I'm bombarded with all these serious sounding emails waing me how I need to register my domain names and pay an extra £50 each approx. Hell I wouldn't have bought those domains if I knew I'd have to fork out another £100 for domains I probably won't even be using for a while. So my question is do I have register them what happens if I don't? If I've purchased the domain isn't that enough? Sorry for the newb questions.

thanks

Adam