if the wiring is in good condition, it may not be nessasary to change it, however, you must ensure that there is an rcd in the circuit, to prevent an electric shock whilst mowing the lawn or using tools etc in the garden.
If you considering, running a cable underground for more outlets or some of the existing cables are crossing the garden and not just entering sockets on the wall SWA is essential under 17th edition, buried at a depth of 600 mm to be safe.
However at all times, all power outside Must have and rcd attached at the dist board.
Agreeing with what has been said I would add that twin and earth is the wrong colour. OK may seem daft but in fact correct. Black will stop degrading due to ultraviolet better than any other colour and you will find all outdoor cable is black. Putting cable into trunking will help but in the main it will take 10 years plus to degrade so normally not a problem.
Since we test the domestic system every ten years not really a problem and unless ID's on a PIR I would not worry.
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