Antiguru has basically answerred the question but I'll add my support to the important additions from Everhelpful and SteveD and add just one more: Don't forget to complete a CN22 (Customs Declaration) form and attach it to the package. If you fail to do so, it's quite likely that the package will be impounded by the US customs authorities.
Surface mail is cheapest but you should be aware that Royal Mail quote a 'delivery aim' of 8 to 12 weeks for surface mail to the USA. (Airmail has a 'deivery aim' of just 5 days).
I don't know how big your DVD box set is but 3 DVDs in standard cases (and including packaging) would cost �1.92 by surface mail and a little under �4 by airmail.
Chris