cheapest way to heat conservatory?

[B4 Sloth]

After last years HUGE electricity bill as a result of having a plug in oli heater on all day I need to think of a cheaper alternative.
The conservatory is the kiddies room, full of toys- not one radio controlled car yet but am working on it

Is underfloor heating cost effective?
What about storage heaters?
Or should i get the copper piping out and add a rad? (Least favourable option).

I know its not an exciting topic but am sure someone will have an idea