In a well insulated space, storage heaters are just the job. They are always available on ebay. I have bought both new and used before. With the used, you can spray them with white gloss radiator paint in rattle cans and they then look like new.
http://www.ebay.co.uk/itm/Dimplex-2...182768?hash=item43ec1dc4f0:g:QvQAAOSwAvJW-9bW
Just get them with a 3kw max input rating per heater, that way you can run them off your ring main using individual timer clocks and normal 3 pin plugs without the need for expensive rewiring shenanigans
The bricks are heavy. But they are very easy to dismantle, install and wire in. If you buy a well established brand (dimplex, creda), parts are readily available and cheap. There are millions and millions of these things installed in the UK so spares will be available for years to come.
Many folk think they are crap/ cheapo as their experience is of storage heaters in a cold, poorly insulated house where it is too hot in the morning, too cold at night. But whilst I know that I am repeating myself, the key is a well insulated room. Then they are perfect for this kind of application.
A friend who is a property developer advised me many years ago to fit these things - maintenance free, no water pipes to freeze if folk go away over Christmas and leave the heating off, no air locks, silent operation, no pump to fail, no boiler required, no annual safety inspection/ chimney cleaning/ servicing, no banging when they come on in the evening, tenants do not steal them as they are not valuable as scrap, no pipes to burst or leak