Utilities Laid In Garden

[wrayperson]

Can you legally lay electric cable, water pipe & drain in your garden if you work in the building trade.

[Buenchico]

If you're connecting up a new electrical supply, with its own consumer unit, you'll need to go through a DNO (or IDNO). See here:
https://www.ofgem.gov.uk/sites/default/files/docs/2014/08/ofg538_web_how_to_leaflet_4_0.pdf

If you're connecting something to an existing electrical supply (e.g. running a cable from your house to provide power in a shed) anyone can do the work but it must be certified by a qualified electrician.

The Water Supply (Water Fittings) Regulations 1999 place restrictions upon who may certify relevant work. The forms and (particularly) the guidance notes here provide further explanation:
http://www.anglianwater.co.uk/developers/new-or-replacement-supply.aspx

Whoever does the work must meet these requirements:
https://www.legislation.gov.uk/uksi/1999/1148/part/II/made