Heat pumps can in theory run 3-400% efficient vs gas which is at best 100% but is probably 85% - 90%.
Therefore as long as electric unit price used for the heat pump isn't more than about 300% of a gas kWh price, then it should be much of a muchness.
The insulation thing would also apply for gas CH unless you're hoping that the higher rated power will overcome poor insulation. In a poorly insulated house I could see that being an issue, but I could also see the current central heating performing poorly as well.
Gas heating is much hotter than heat pump. It's why u can have got hot water water on demand. It takes longer to heat up a room with heat pump hench why it's advisable to fully insulate your property to maintain the desired temperature.
Insulation will of course help with gas CH as well but if you compare heat pump vs gas CH on a none insulated walls and ground floor under floor insulation, the CH Gas heating would perform better and cost less to run.
Now add in solar panels and you will start to see the true benefits and then insulation.
But again, solar panel is not a DIY job so someone needs to install that and insulation of external walls could be done from inside with advanced DIY but outside you need a pro.
And then there's the hot water cylinder for on demand hot water especially /ideally needed for a small family of 3 for example.
It all adds up and all needs to be considered.