It's not just currency issues.
It's more about the lack of transparency of many financial markets around the world. Overseas carrier play the handset subsidies better because they don't have to file detailed SEC quarterly filings.
UK carriers like to do huge discounts on prepaid phones during christmas to boost their financial numbers --- guess what more than 50% of them are box broken and exported overseas. The carriers are chasing fake customers --- and they don't have to give you any detailed financial filings on them.
UK is full of box breakers, cashback deals (aka pyramid scheme) and VAT carousel frauds. It's a dirty game in Europe and I hope that American companies don't that game at all.
Anoter person who has no idea about the UK market and maybe the EU in general.