Duties are not charged on anything that costs less than £18, including postage. So a CD would get through without any problem. I have received books, but then they are zero-rated on VAT and probably don't have duties anyway.

Anything more than that, you are expected to pay Duties, and VAT where appropriate. Plus the Post Office will charge you £4 for having to deliver the bill to you and collect etc.

To some extent it is a random charge: the Customs and Excise do not check every parcel coming into the country. At times of drug or terrorist hype then the percentage checked goes up.