It will cost you surprisingly little to get this scheme designed, installed, tested and commissioned properly as long as you are prepared to do the donkey work (ie digging a 1 metre deep trench from house to shed and doing any fiddly wiggling at the house end to get the end of the SWA to your consumer unit). You'll probably get a better deal buying the SWA to your electricians' spec than letting him/her supply it but get a price from them anyway.
While you've got a big trench available throw some ducting in it (for CAT5, alarm, telephone, any other odds that you might want at the end of the garden in years to come).
Cost wise (unless you are going mad with sockets at the shed end) you're looking at a days wages for the spark plus £200 in bits and pieces (excluding the SWA). Re the sockets in the shed- your electrician may suggest using conduit, singles and metalclad sockets rather than plastic sockets and T & E- takes a bit longer but is easier to get neat and you can pre prepare the conduit and backboxes thus saving their time (and your money). Either way you get a properly designed and tested installation notified to Building Control into the bargain (as long as you used a Part P accredited electrician of course).
