If the OP has, as he claimed, passed his 2382-18 then he should know that the sockets need to be protected by a RCD; if he's his Part P, then he should know the job will need notifying first; the source may be an addition to the house circuit but it's a new installation into the detached garage, so he's putting a new installation in a 'Building Sharing a Supply with Dwelling' and a 'Detached Shed, Garage or Greenhouse'
His suggestions of using steel trunking in the ground (where it will rust away) and of using T&E in trunking tells me he shouldn't be doing any electrical work.
"However I checked his ring and there is not much on it." doesn't mean a thing, you have to calculate the circuit load allowing for diversity, after all the next occupier/next year, may have much more attached to it.
He says there's no water services to the garage but does it have ANY extraneous conductive parts?
This, as has been said, should be done properly as a sub main from the house CU to a GCU. It needn't mean lifting 40 slabs as the armoured cable could be clipped on the outside of the house wall/under the eaves etc, to the point where the OP is thinking of routing to the garage; or a 6mm T&E could be run into the roof void, across to that point, then junction to the armoured to come down the wall (e.g. behind a drain pipe?).