If it’s just a heater, light and a couple of sockets and the summer house isn’t going to be 15 miles away, them probably a piece of 6mm sq 3 core armoured cable, laid in at a suitable depth (ideally 450mm where anyone would be digging, but must probs go to about 300mm 😂). Is your house distribution board on an outside wall? I’ll take some pics of mine shortly
So I’ve a 50mm conduit and pull laid in already when I had the water pipe run so that’s easy, but that needs a cable running internally in the house, to the conduit, then 30m to the summer house, so 6mm be way over for what’s wanted!
It’s the dragging it through my walls and into my distributon box that really I want a spark for because, frankly, I don’t like dirty work 😉