When I decided to add a similar opposite-side socket in my kitchen, I found that the easiest way was via the roof. Needing to keep the wire away from the existing polystyrene insulation actually gave me a good solution, I pushed a length of garden-hose up from an existing socket to the corner of the roof, then another straight across to the other side (easy because it went alongside a support), and a third length up from the new socket to the roof. The hosepipe proved remarkably easy to push, as it didn't kink or bend sharply, and then acted as an excellent conduit into which to push the wire itself.