Because it isn't mildly long. It's either very long or infinite, anywhere where gas is ~ 1/3 the cost of electricity per kWh. COP of 3 at a 3:1 price ratio for electricity to gas = 1.

Which itself makes sense if you use gas as the RELIABLE source of kWh. You need to burn ~ 3kWh of primary energy in gas to deliver 1 kWh to the household, so 3:1 makes sense, and the RELIABLE source of kWh is the price setter because a failure of the grid is literally an existential crisis, something the "agile" private sector companies aren't liable for.

An auxiliary annoyance in the UK is that reversible pumps aren't renewable for government grants. This is ostensibly to discourage aircon, but rather silly, since there are few easier green power problems today than "satisfy a demand that is almost perfectly correlated with high insolation".

FYI as a type the largest suppliers are gas, solar, then nuclear - of which nearly half is being pulled from France, an amount that for some reason isn't always counted by some people.