How does uk customs work?

[Nirvanafan]

I sent or a fan mail to Andrew lincoln from the states to the uk and was just wondering how uk customs deals with mail? Do i have to worry about them opening my stuff or rejecting it? I did fill out the slip at the post off but how do you put a price on some picutres your printed out. any ways just want to hear back from you guys who send to the uk

Thanks