British plugs (BS 1363) are used in hell a lot of countries besides the UK: So the device can come from any of those places / was built for some of those markets. In addition to that, a very similar older British standard - BS 546 - is still widely used in India, Sri Lanka, Nepal, Pakistan, Nambia, South Africa, Hong Kong, Botswana, Israel, Singapore and Malaysia. There is no British-targeted firmware available for the N9 so I guess your device was built for one of those target markets. The best way to know that is to check the product code under the micro-SIM tray and match it against the available firmwares. P.S. All online stores selling the N9 in the UK claim that they are selling mainland euro version with an UK plug adapter, so quite similar to your situation but just in reverse.