You're asking a question that tends to have a LOCAL answer, even here in the USA.
In the US, operating costs tend to be, low to high, like this: solar => heat pump => natural gas => propane => electrical resistance heat. Equipment costs, again low to high, tend to be: resistance heat => solar => gas/propane => heat pump.
BUT, solar only works where the sun shines, so it does not work as well in Washington state, Oregon, OR England, as it does in Arizona or Southern California. Heat pump only work well when air temps are above 45 degrees, so they work better in Florida and Louisiana than in Michigan. Gas rates have come WAY down in the USA, due to oil shale fracking, but I'm not sure that the same is true in England.
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