Gas boiler or immersion heater ?

The sensible option is to fit a time switch to the immersion heater so it comes on twice a day, probably for no more than an hour each time.

A gas boiler coming on twice a day means it is not a combination boiler (which is always on for hot water). That means running the boiler long enough to heat stored water. Gas boilers are relatively inefficient so you waste a lot of energy.

A immersion heater on all day will maintain the water at a high temperature all the time, repeatedly cycling as the water cools a little. Because the water is maintained at a high temperature, the heat loss will be constantly high so it will keep re-heating and using electricity, depending on how good your cylinder insulation is. An immersion heater is very efficient but leaving it on all day is very wasteful of expensive energy.

If you set an immersion heater time switch to heat the water before you get up and again later in the day, you can arrange to have hot water when you need it without wasting electricity. If you use all the hot water and expect to need more before the next programmed time, some time switches allow you to do a one-hour boost.

If you are using, for example a combi boiler, for hot water/central heating, then you will only heat the water that you are using at that time. If you are using an immersion heater then, in effect, you are storing hot water. Leaving the power on, without using the water is expensive. I suppose solar panels would supply the electricity, but panels are costly, and you would need a lot of them! Gas prices vary enormously; different tariffs, different companies, so you would have to do your homework carefully. My preference is gas, mainly because I can control the usage more effectively.

Took the fuse out of my immersion heater years ago.

Gas fired boiler wins hands down.

boiler, hands down