If it's going outside into the garden, which is a special location under the regs and Part P, and being run from an existing circuit it will require a minor works certificate and notification to building control - because it's in a special location. Whoever notifies the work should be Part P registered.
To fulfill the requirements the light must be 30mA RCD protected and earthing and bonding should be to the current requirement of BS7671:2008(2011). SWA burried in the ground should be to a depth of 600mm where anyone may be likely to dig, with a warning lable approx 150mm above it. Ideally the cable should be laid on a bed of sand and also covered over with sand before backfilling. If the cable is going under hard standing ground (patios, paving slabs, concrete, tarmac etc) it is reasonable to reduce this measurement to 300mm. If you are connecting to an existing socket circuit that is not RCD protected I would use an RCD spur switch with a 3 amp fuse and run the cable through the wall into a Wiska box. (Make sure the cable is protected by if it crosses the cavity.) Terminate the twin and earth with the SWA there and don't forget the earth link and the banjo. It's easier than taking the SWA into the house. Also drill the drain hole in the box where it is marked otherwise damp may collect there.
If your circuit you are connecting to is already RCD protected then do exactly the same, just use a normal DP spur switch with a 3 amp fuse.
Good luck
