In Nokia phones, UK English is used as the source language for most languages, and this very same source English is also available in phones sold in various regions. If you think your application will need localization, using UK English as a source language is recommended. It is relatively easy to acquire translation services (or use crowd sourcing) for texts written in English. This may also be less costly than using a different language as the source.
If your application is specifically targeted at the Americas, there is a good reason to select US English instead. If you intend to have your application in one English only, for various regions, you need to keep it clean from expressions that are strongly linked to specific regions or cultures. Idioms, names of sport or food, for example, may need to be avoided. Once you have selected the source English variant, stick to it in all parts of your application.