At 6kW you'll almost certainly need a separate circuit and isolation switch off the meter (unless you have a recent 17th reg consumer unit with a spare unprotected way) and a secondary split load distribution unit with 2 rcd and separate power & lighting circuits in the shed as you certainly don't want to lose the shed lighting if the power circuit trips. You'll need 4mm SWA cable as 2.5mm is marginal on voltage drop.