If you qualify for them and if they are campaign/gallantry medals then you can wear them, if they are for achievements such as Neimegan (sp) march then they can be worn on the opposite breast
I have the 1939/45 Star, the France and Germany Star and the Defence Medal. I bought a Bomber Command Commemmorative Medal which I can wear below the official ones.
I've got two medals issued by Saudi Arabia and Kuwait to me from the Gulf war in '91, but I wasn't allowed to wear them while I was in the Army (unlike our Yank cousins who wear them in uniform). They are still in the boxes, never been worn