I don't think it ever makes sense to switch to electric radiators. It might make sense to switch to a heatpump, but you need to avoid being hugely overcharged by the installers and there are flow issues if you have 8mm piping.
This ROI calculator looks reasonable: https://ukcalculator.com/solar-panel-roi-calculator.html - note that it subtracts the install cost for you, so any case where the final figure is positive is profitable. But of course that depends on whether grid prices go up or down in the next decade ...
This ROI calculator looks reasonable: https://ukcalculator.com/solar-panel-roi-calculator.html - note that it subtracts the install cost for you, so any case where the final figure is positive is profitable. But of course that depends on whether grid prices go up or down in the next decade ...