You'll need to work out the weight (times weight per CD/mag + box) then check Royal Mail prices here:

http://www.seajays.org.uk/postage/

Or couriers on here:

http://www.interparcel.com/quote/

Interparcel supply their own tracking numbers which can get complicated if you need to claim so it's best to book direct with the courier you decide on.   Just Google for their website.

Or these are good for large/heavy items:

http://www.shiply.com

Put in the details and they'll send you some quotes to choose from.    The firms involved may not offer tracking so check that for seller protection.  Not necessarily a problem but worth bearing in mind.