My real car's engine temp

[Big G]

I drive a mk4 golf TDI and in the normal UK weather the engine temp sits just below the 90 degree mark on the gauge. The Mrs has exactly the same car, but her's has AC and mine doesn't and her's sits bang on the 90 mark.

Now the snow has arrived it's dropped to probably the 80 degree mark, but when I use the heats on level 3 the temp drops to probably 60 degrees. If I use the heaters on level 2 the temp goes back to around 80.

Is this normal or is my antifreeze a bit off? It's never frozen solid, but I haven't ever changed my coolant since owning the car lol. I've had it for almost 3 years now. I'll get it checked anyway and replace it needed.