Gas versus Electric heating for boiler in UK - Which is cheaper?

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I have got a question. My family and I have been staying in UK for 3 months now.. and it is getting colder each passing day. The house we rent has a boiler which can be heated by gas as well as electric (as a backup). Would it be cheaper if I were to use electricity to heat up the boiler or gas. I asked because gas price has been increasing steadily of late and I am not sure now if it makes anhy difference between that two. Could someone advice me on this. Would apprecaite it if you could enlighten me of the price differences as well (huge, marginal or insignificant difference). Thank you.