Regulations for buried cable in the UK
as I understand it BS7671 requires that the cable is buried deep enough to prevent the cable being disturbed by any reasonably foreseeable disturbance of the land but does not specify a particular depth. There should also be a warning tape burried above the cable.

Normally though the recommendation for a domestic garden is around half a meter. That is enough to give it some margin beyond the depth of a typical spade. 

If you will be be getting an electrican to connect up and certify this I would strongly advise talking to them first before you buy and install the cable.