I think there are two choices:
0) Electric. Fully thermostatic, and as such I think the cheapest for Spring frost protection. Only on when needed, and apart from any worry of power cuts?? no need to visit greenhouse of an evening to "light it" etc. Cost of fitting an armoured cable to the greenhouse, and a mini distribution board and an earthing rod is ... significant and, of course, a non-starter for an allotment or site remote from house.
1) Bottle gas. Runs on a pilot light, and a crude thermostat, so not too wasteful. produces some condensation, thermostat is crude so Max/Min temperature is a bit of a guess. If you already have a bottle, for BBQ/similar then only cost of fuel, if not there is cost of the "rental" of the bottle. Need two bottles to be able to switch over when the primary becomes empty. Dunno if there is much chance of the pilot light going out?? Risk of the Bottle being stolen on an allotment?
2) Paraffin. Lots of condensation, risk of it sooting everything up if the wick burns unevenly, need to light it manually on cold nights and extinguish in the morning, so it will be burning all that time, regardless of whether the temperature is/remains cold or not, so expensive on fuel.
Either Gas or Paraffin can be put under metal staging and will heat the staging, and thereby the roots of the plants, which will enable them to endure colder air temperatures.
You might get away (for a small enclosure, within a greenhouse) with Tea-lights - an upturned clay flowerpot over the top will help to spread the heat. Has the drawback of having to light them in the evening when cold temperatures are expected (although no need to extinguish them in the morning I suppose?!!)