summerhouse electrics

I ordered a new summerhouse yesterday, which I will use for my clinic for massage and Reiki, and eagerly await delivery in 14 days time Whilst fully aware of the new regs for qualified electricians to connect the supply, the cable will be put in situ ready when the base is done. Can someone please advise which armoured cable I need to buy. It will be about 30 metres from the house, run overground, through a flower border, alongside the perimeter fence. I will need lighting and one, maybe two, ordinary 13amp double sockets. At a later stage, I hope to have some low-voltage garden lighting and a water feature at that end of the garden too.

4-6mm cable should be plenty for your purposes.

Yo - 6mm² minimum. 10mm² better, as the marginal cost difference overall will be negligible, and if you want a sauna or a spa, you'll be able to do it. (Don't advertise sauna & massage though, or you might attract the wrong sort of customers... )

But on a serious sauna note - I don't know if you've been in any summerhouses in this weather, but you should give serious consideration to installing aircon - it'll be difficult for your customers to enjoy their massage or concentrate on their aura if they are lying in a pool of sweat...

If its a business premises, then part P is superceded by electricity at work act.
Most important however, lay the cable in such a way it will not be damaged, usually this means buried 18" deep or more, in soil or sand free of sharp stones, or clearly visible, either overhead, or clipped to a masonary wall.
As for the bit where it comes up from the trench through the concrete base, putting it in some a pipe or tube to keep the cement off will make life so much easier if there is ever a problem that requires a new cable re-threading. If you may ever want a phone line lay an additional conduit in for that at the same time.
At 30m, for voltage drop reasons and the effect of flickering lights when something is switched on, I'd suggest err on the side of fatter rather than thinner. At least 6mm prefferred - and get 3 core, so the earthing is augmented by the armour, rather than depending on it. If your supply is PME you may need an earth rod, and associated green yellow tail wire threading into the outbuilding too.