Motorway rapid charging in the rain - is the Model 3 heat pump just burning my cash?

Ey up. Been driving the Model 3 up to the Dales this weekend and the bloody rain is relentless. Noticed the heat pump is working overtime and my range dropped like a stone on the A1(M). Stopped at the Gridserve in Ferrybridge and paid 79p a kWh for a top-up. Absolute daylight robbery, lad! Back home on my Octopus tariff it’s pennies, but these motorway stops are bleeding me dry. Is the cold wet weather really killing the battery that much, or is the car just being greedy? Anyone else finding the M1 corridor chargers are an absolute rip-off lately?