Likely bottled gas and a gas fire it cheapest to run, can get oil fired space heaters but tend to go for silly prices, but safety wise and installation cost electric may be better.
The speed it heats is important, one is allowed an hour, but really you want warm within ¼ hour, fastest is inferred, then the fan heater, then convection heater, then panel heater, then oil filled radiator, the next consideration is safety, plinth heaters although really fan heaters are mounted where unlikely to be damaged or anything fall on it. Oil filler radiators are reasonably safe, seen a few fires with convection heaters where paper has fallen on the heater, or mounted upside down to cook sausage rolls on.
The common sense of people in the office is important, fan heater is designed to be flat on the floor, have seen a fire due to being mounted inside desk space to keep worker warm at 45° blowing down on their feet so sensor to switch off if over heated in wrong place.
I don't think I can say any heater is 100% safe, even the oil filled radiators found one where the thermostat bolted on the end bolts were loose so main section not earthed.
the gas heater with built in gas bottle is around £95 and 4.2 kW so likely the cheapest easiest way, but it will make the room damp, a version with a flue much better, made mainly for caravans and boats, this one
uses 35 sec gas oil, and does have the flue outside, and using blow hot air is fast.
Many of the caravan and boat heaters are technically not permitted for use in a building, due to size they do not comply with efficiency regulations for building heating, so technically you need house type central heating boilers converted to use bottled gas or oil fired, but oil and gas even bottled gas is so much cheaper than electric although designed for boat or caravan they are much cheaper to use than electric.
May not look the part in an office, and so may not work, but far cheaper than electric to run.