Woodburners might be ok in the open country but they are increasingly choking up our villages, towns and cities. The clean burn type approved for smokeless zones might be ok but homebrew things are generally filthy. Maybe you don't care, but you should.
The B&Q '27 amp outdooor armoured cable' at 1.5mm2 will do about half that (14.5 A) or around 3.5kW at 25m with under 5% voltage drop.
Why does everyone and his dog seem so averse to lots of insulation and reducing heating requirements to the absolute minimum (or nothing) ?
Storage heaters force you into a regime where your daytime electricity costs for everything else are significantly higher, for the dubious 'reward' of lower cost heating you may not even require the following day. Even with 100% occupancy they are evil antiquated things, always have been, always will be.
With a suitable south facing wall a solar air heater can be knocked together by a complete amateur in a couple of weeks.
Have both a solar heater and significant levels of insulation and you'll never need a gas, electric, wood or coal fired heater, leaving more money for workshop machines
A few links on solar air heaters:
https://en.wikipedia.org/wiki/Solar_air_heat
http://www.builditsolar.com/Experimental/PopCanVsScreen/PopCanVsScreen.htm
http://www.motherearthnews.com/diy/build-an-inexpensive-solar-heating-system-zmaz77ndzgoe