The heat pump is for heating in winter on longer journeys instead of the resistance driven heater, it means that you use about 1/4 of the power for heating the cabin. Many EVs have heat pumps as they are much more efficient, but models sold in countries with warmer climates just get the simpler cheaper heater/aircon instead. Living in Scotland you’d likely benefit more from the heat pump than you would living in the far SE of England, it’s a balance you need to work out for the BMW i3, how much range do you want to sacrifice to the heater…
No need for it on short journeys.
No need for it in the summer.
Marginal need for it in parts of the UK, but you’d want it anywhere in Europe that has the colder winters.