Try the IET
http://electrical.theiet.org/wiring-regulations/
Hence BS7671:2008(2011)
SWA is the way to go, you have a spare way on a 17th ed CU, like Jonny round boy says pop the manhole cover to check the easiest path for an underground run and then decide how much will be underground v along the wall. If you're going to do the digging yourself. As stated 500mm is the minimum but there's a lot less fuss if you can bury a large pipe as a run. Direct burying SWA will involve sand above and below to prevent damage to the sheath from stones etc. Either way put warning tape about 300mm down for the entire length and take a photograph of the route for future reference.
Do have a small DB / CU placed in the shed, Schneider / MK / Hagar there are plenty of good brands. IMHO the quality of the brand / ease of installation matters most, personally I favour TLC as a supplier (service with a smile) but even Toolstation can supply a functional plastic conduit box!
A small CU with it's own RCD and 2 to 4 ways gives you one MCB for a socket or two another for lighting as well as 2 spares ways for the future.
As others have mentioned make sure your earthing is checked, with properly installed SWA there's a good likelyhood you'll be able to rely on the existing earth provision but make sure.
Do have someone who can legally issue you a cert and do make sure it's someone who actually tests what they say they do. Which applies regardless of what scheme or quals they have! This means they will have to do the cable installation / test / commisioning to sign for it. But you can still do much of the planning and preparation esp groundworks. Do overestimate your future needs a bit and build it in.
With your catV! run a separate plastic conduit make sure it's glue jointed to seal it and use gaskets / glands at covers / ends and either run 2 or 3 lengths straight away or make sure you can pull through if you need to, terminate both ends at shielded sockets and pay great care not to compromise the sheilding. One tip is to use grommets for entry holes even if the back box is plastic.
One last question: how far will you have to walk from your armchair to fill the kettle?
EDIT> personal attack towards other forum member removed.