... in the UK it is very common for us to have to pay more simply as it's the UK when there is no actual economic reason to.
No actual economic reason? Yes, there is: it is simply known in Economics 101 as DEMAND. As long as you folks in UK do keep buying at those "outrageous" prices, there is the demand, and the companies would be crazy to sell you for less. Free-market solution: stop buying and open up the market for competition. Socialistic solution: cry mommy, run under her (the government's) big skirt, and continue blaming everybody else but yourself (and her).
... Companies do it because they can get away with it...
You finally got one thing right. And they can get away with not because they have not been caught by your Big Mommy yet, but because
you keep buying at those "outrageous" prices. ... I and many others are fed up with that corporate thieving attitude...
What you call "thieving", I, as an investor in those corporations, applaud as a smart business practice, increasing their profit and my investment return.
... Can you really justify Brits paying up to 100% more for an identical product...
First of all, there is no reason to justify prices in a free market: producers are free to come up with whatever price they want, and the market will tell them if they were wrong; if they can sell at such a price (like in UK), they were obviously right.
But, since you seem to prefer the Big Mommy approach, I will try to offer an explanation (not justification):
Selling identical product at different prices is known in Economics 101 as
Price Discrimination. Contrary to popular belief based on the use of the word "discrimination", it is not illegal to price discriminate. On the contrary, it is widely in use and also taught in every business school as a preferred weapon of choice in maximizing profit. Two conditions for price discrimination to work: proper customer segmentation (i.e., defining customer groups willing to pay more, e.g. UK customers, or business travelers) and ability to put up barriers between those groups.
UK companies price discriminate inside UK every day. When was the last time you flew and all the passengers on the same plane, departing from the same airport and arriving to the same airport, paid the same price? Among the economy class passengers, the airline will discriminate based on the proximity of the ticket purchase to the flight date, for instance. As for the business class... when you flew it, did you really feel that the service you got is 10x better than the service a coach traveller is getting? And yet you paid 10x more for the service that is perhaps marginally better (sure, you get metal fork vs. plastic). Why? Because the airline was able to segment you as a business (or affluent) customer, i.e., capable (or willing) to pay more. For some (affluent) the metal fork is the [price discrimination] barrier, for most others it is the required Saturday night stay.
Alternatively, forget Photoshop, switch to GIMP (a free clone). Ooops... I almost forgot: you do not want the lowly clone, you want the real deal, the best product on this planet the mankind has come up with, and you want it at the price
YOU consider right... and of course you want the Big Mommy to force Adobe to sell it to you at that price.
Or how about this: come up with your own (i.e., UK or EU) Photoshop equivalent at an "outrageously" lower price, and I will be happy to buy it from you (even if you price it higher for the US market, but below the current Adobe price). Oh, no..., I forgot again: you prefer to invest your time in dragging foreign companies to court, rather than investing in research and development of your own.