Something that would really help the English variants is to have a feature that can copy all of the original strings for untranslated as the translated string, and leaves them as Waiting status.
Being able to check and bulk accept these strings (fixing variations where necessary) would be considerably quicker than going through each string one by one to click on Copy original and accepting.
~95%+ of strings are going to be identical anyway between US English and British, Canadian and Australian etc. English, so I think it’s sensible to go with the assumption that strings match, and change where necessary, than assume the US English is going to be wrong (as with all other non-English variant languages).
There appears to be several strings missing from the .pot available for https://translate.wordpress.org/projects/wp/3.5.x/en-gb/default according to the results returned from a search.
WP Core contains strings like “This comment is in the Trash. Please move it out of the Trash if you want to edit it.”, “Trash Comment”, “This comment is currently in the Trash.”, and “You are about to move the following comment to the Trash” etc. but the search for the term “trash” with Any status (https://translate.wordpress.org/projects/wp/3.5.x/en-gb/default?filters%5Bterm%5D=trash&filters%5Buser_login%5D=&filters%5Bstatus%5D=either&filter=Filter&sort%5Bby%5D=priority&sort%5Bhow%5D=desc) doesn’t show these.
There well may be other strings that aren’t listed for translation, but this is all the detailed digging I’ve done so far.