legality of buying online

Hey all ! i am a newbie, so I am sorry if maybe I am posting this in the wrong space..! But I wanna ask if someone knows if, according to UK law, is legal to buy some medicine online? i am reading some discussion and there are lots of people who are buying online but anyone is saying anything about the legal aspect of that. I' ve noticed you can get medicine that require usually a prescription without it..is it legally possible? I don't want to have any problems with the justice, so if anyone knows would help me a lot!!any experience would be more than welcome.