Depends on your set up if you invest and generate your own electric via PV with batteries then the cost can swing the other way you can sell power back at peak demand for a good return as well as powering your own home but it's not cheap , kop
Depends on the boiler. Gas is cheaper to buy, but I think my old boiler is about 25% efficient when in water-heating only mode, so probably better to test by setting up a scientific experiment and reading the meters. Immersion 100% efficient, but electricity more expensive.