You do know don't you that you have to be certified to legally do wiring outside of the house?
If you're not it has to be inspected by building regs.
These guys make ready wired lengths of armoured cable which they claim makes installing them yourself legal.
Personally I'm not too sure how true that is but it's certainly going to be easier and safer than doing it yourself.
A fiend of mine is an electrician and did ours, apparently wiring up armoured cable properly is a bit of an art .
Incidently if you bury it you sould lay warning tape in the earth above it so that anybody digging there later will find the tape before the cable.