First idea that springs to mind is price discrimination. The US and UK are two different markets which have different PED's (Price Elasticity of Demands). PED basically is the extent to which the demand for a product is affected by a shift in price. If the demand is highly elastic, it will be affected a great deal by a change in price, if inelastic, it will be affected to a smaller extent.
Price discrimination occurs when one firm is operating in two separate markets with differing PED's. The assumption is that buyers in the british market are more willing to pay a higher price for the product than buyers in the American market and the firm exploits this inbalance to provide the good/service (in this case the magazine) at 2 different prices to maximise profit.
In other industries where goods are sold at a higher price, for instance cars, there are difficulties with price discrimination as products sold in one market for a cheaper price often tend to leak back to the market in which the price is set higher as people import them to save money. In the case of a magazine, this is hardly plausible as importing costs would outweigh the savings made by buying in a cheaper market.
Like I say this is just very basic economic theory and I'm not saying this is necessarily what PGi are doing just this is an economic explanation for this kind of thing.
Yeah, that's it. It's got nout to do with the fact that it hasnt been changed since the exc was 1.75 and the sub manager has been fired.