Ooooh, this is something I know a bit about! Being an Ex postman.
Posting it should be very easy. As said above, just turn up at the PO and it'll all be done for you.
You can send it by Airmail, if it is 2kg or under. Though you may find that sending it as an International Parcel will be cheaper, though somewhat slower. You could even send it "surface" if it is non-urgent, as this is by far a lot cheaper. (one trick though, to send it surface, that works about 50% of the time, is to pay for the surface rate, but put an Airmail sticker on it. Most posties don't know all the International postal charges, they just throw it in with the rest of the Airmail stuff!)
Another thing to know is to write the words "Small Packet" on the item. This will dramatically reduce the price. If you are sending books or newspapers/magazines etc, write the words "printed paper" on the item for a further reduction.
Example.
Normal price for 2kg airmail item to Europe = £18.64
Small Packet @2kg = £10.88
Printed paper @ 2kg = £10.60
Royal Mail's website has a range of options to choose from.
Here's the address for Airmail and Surface mail.
http://www.royalmail.com/portal/rm/jump1?catId=400023&mediaId=400033
For Parcels, they'd go by ParcelForce (aka ParcelFarce), which is a Royal Mail company, and you can find out about the cost for your parcel at the PO. You might also be able to use RMSP (Royal Mail Standard Parcels) which ISN'T ParcelFarce, though I 'm not 100% sure if the service is International... but it'd be useful to ask.
Hope that has helped.