Is there that much choice? Presumably mains gas is out, which just leaves bottled gas or electricity - or Pink Paraffin - or Esso Blue. (do they still do that stuff?) Just remember (as eluded to by owain) that small electric fan heaters convert 100% energy to heat where expensive 'super efficient' expensive electric heaters convert 100% energy to heat. So don't spend money on anything expensive.
a 2-3kw fan heater will be the most efficient at heating the space if used properly
3 kw will heat it up 5 or 6 times quicker than a 1kw dependant on heat loss
you can aim the heater at your working area like a desk and you will get direct heat
a fan heater will also react to the thermostat much quicker with no time lag over an oil filled heater
but keep in mind a 1kw oil filled radiator can only use say 15p an hour no matter how much it struggles and fails to warm the room
whilst a fan heater will cut out at temperature but can use 30-45p an hour getting there
other things to consider it other things using power and supply available to the room
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