I do understand OP's problem. What you need to understand is that currency conversion is not made by ATA app (be it KaW, PIMD, MEEGO) but by iTunes purchasing system. That extra pound that OP is paying more for the 200nobs pack is going in Apple's pockets not in ATA ones. ATA is getting 70% of the listed price in USD. That's $14 for an $19.99 pack even if actually Apple is charging OP with $21.44 for that pack.
So wrong....oh so wrong.... I'm on UK Android and I see initially the dollar amount and the converted UKP amount! OP seems to have gone quiet so tomorrow at work I'm going email ata and see if they are financially clueless when it comes to internal exchange rates (btw I'm a financial consultant working on a global finance software package so I do know the difference between external exchange rates set my companies like google and internal rates!)
Here is my "Oracle" prices in euro. Here is the purchase dialog. As you can see the currency conversion is made by Google Wallet purchasing app and not by KaW app. Same goes for Apple. The only difference is that after currency conversion Apple is applying their price tier system. That price tier can be lower or (more often) higher than converted value. Here is the explanation for the one who posted couple of pages ago related the price difference between his iPhone and his wife Android. That 6xtals pack cost $5. $5 equal with 4.15euro. The 4.15euro amount is between 3.49 and 4.49 euro tiers and Apple is placing it on 4.49euro tier. But this can go the other way around too. The system is flawed and can happen (not so often) to be lucky and sent in the lower tier. The only thing that ATA can and should do about OPs issue is to ask explanations from Apple why those two $19.99 worth items are placed in two different price tiers in UK Store.
good for you, but honestly not interested in your personal backstory... you clearly didn't read my post properly since I said UK iDevices only show GBP... I mentioned nothing about UK androids only showing GBP. FYI, I have a UK android device as well, yes it shows USD first and then the conversion...my device also shows 200 nobs and 32 xtals at $19.99 and both convert to £12.74 (Amazon Store).
You see if i had 4 sticks and sat on one of them , now how many stick do i have, wait cheese hamsters, oh yes i got it, 20 bucks ok cool. Thanks for the fish sandwich God.