Sallywally1 Lurpak is what is known as lactic butter. Traditionally, lactic butter was made by allowing milk to sit for several days, encouraging the formation of lactic acid which ferments the milk into a yoghurt-like consistency that could be churned into butter. Ordinary butter is made from milk straight from milking.
Nowadays lacticc butter is made by adding lactic acid to the fresh milk to encourage it to ferment. I understand that it is far more popular in he north of England and Scotland than further south.
I suppose it is more expensive because there is more processing involved. Personally, I do not like lactic butter, preferring the straight 'cramery butter', but then I am a southerner.