From a commercial point of view, Calor do not produce LPG, they buy and bottle it, so there is going to be some mark up. Butane gas is sold mainly for the leisure market such as camping and caravanning, so this is a niche market where the predominat supplier can charge basically what they like, and make a healthy profit. Calor is also available at many camp sites, so it is also the cost of convenience.
By comparison, BP actually produce LPG. The largest commercial lpg market is for Propane gas, so there is an economy of scale, The big DIY stores that retail the BP products have far more leverage and can force the retail price down.