Check out EPiServerFramework.config. There you can assign a address a default language. Set it to "en-GB" and it should not display the postfix.
You can also set this in admin mode (it simply changes EPiServerFramework.config for you):
Admin mode > Config > Site Information > [Click your site ID] > Edit button > Set the culture
Cheers guys this looks exactly what I need.
I'm building a multi-language site which will have English and Welsh versions of various pages. English will be the default language and I would rather not have all URLs updated to */en-GB/* so by default not show the language code. I'm happy that the Welsh pages would be prefixed with /cy/. Is this possible, I'm wondering if I can use rewrites or something.