1) 1mm² cable is too small to be spurred off a ring main, as the fuse/MCB rating is way in excess of what the cable can safely carry.
2) 1mm² cable is too small to supply the load of a double socket and the lights you already have, let alone adding substantially more load to it.
3) You still haven't said what type of cable it is - ordinary Twin & Earth, or armoured?
4) When you say it goes underground - how? Just buried? In conduit or ducting? How deep? Under what?
5) You still haven't said whether this circuit, which could reasonably be expected to supply equipment outdoors, has RCD protection or not.
6) You need to forget about fitting a security light for now, and concentrate on putting right what you already have. If you do a search here for words like shed, garage, outbuilding, outhouse, outside, and you'll find lots of info on the correct way(s) to run power to your garage.