Windows 8.1 supports both English (US) and English (UK), unlike Windows 7, which pissed alot of us Europeans off, because we don't like spelling realise "realize", personalise "personalize" etc etc you get the drift.
There are 2 different outcomes to your scenario
Outcome A - the ISO is based off of English (UK)
-you are good to go
Outcome B - the ISO is based off of English (US)
-in order to get English (UK), go to Charms->settings-->region and language)-->language--->Install languages--->English(UK), it will download and install, then go into the English(UK)submenu, click "make as default" , it will tell you you have to sign out, just restart your PC, then go back to charms-->settings--->region and language, then delete the EN-US language pack. You are good to go.