I work for an international company, selling industrial things. We have a thing called GRP, for Group Reference Pricing. It's designated in Euros, and it's a reference level for the average price of something. But some markets can tolerate different pricing, depending on demand and competitors and all that.
Some types of our products are very popular in europe, there's a huge market and lots of competitors, so the units sell for less than the GRP. Here in Aus we don't use those types of products (to do with the weather and the way houses are built), noone sells them because noone wants to buy them. But the very few that we do sell, after converting exchange rates and all that, sell for two times GRP, because we can and the market will tolerate it, we're not going to get any more business by dropping the price so we don't (and it's really a hassle and work to sell non-standard things, so higher prices sort of cover the extra work).
For a different type of unit, there's a lot of local competition from well established competitors here, we had to drop our selling price to well below GRP just to break into the market. Over in Europe, they sell for well above GRP because over there we are the established name of quality, we charge higher there because we can get away with it.
I could relate to you my entire economics-degree worth of explanations, but in short:
UK charges higher for film because they can (dropping prices is not going to increase revenue), and they have to (local wages and taxes and rent and all that).
Film is cheaper in the US because they have to (charging more would lose customers to competitors), and they can (taxes, wages, and volumes mean they can get away with smaller margins on bare products).
edit: seeing as someone mentioned that it's cheaper to purchase from Freestyle than from the UK: exactly. There is no UK equivalent of Freestyle (or B+H, or Adorama) in the UK (at least, that I know of from the other side of the world).
Freestyle etc are cheap because they've got a huge operation set up, they ship more units so can tolerate smaller margins on each unit. Compare that to your average corner store, they can't compete with that, they have to have a higher margin per product just to pay wages and rent.