Bury SWA at a depth suitable for the likely use of the ground.
450mm depth generally, you may want 600mm depth thro a flower bed or
cultivated area.

Cable laying method is important.
Clean the trench of stones, lay riverwashed sand bed, lay cable on
top, cover with more riverwashed sand, tamp down, then yellow cable
marking tape with some sand on top, then fill in. TLC direct do the
yellow cable marking tape by the cut metre. Riverwashed sand is used
so a) no stones b) provides a colour change to help identify the
cable.

You can bury SWA in a flexible underground duct.
Perforated land drain pipe (80mm) is about £25 delivered for 25m;
proper twinwall (smooth interior) electrical ducting is much better,
much stronger - but about £75; flexible nylon or polypropylene conduit
(not PVC) of 25mm would give a little protection against exploring
spade although not much.