en-gb messages

So how does that reflect in the translation status... it would never get to 100% and it would be harder to see when strings do need translating again without reviewing every one? Is there a method to mark the rest of the strings as "same as source"?

You have a good point but I think most translators just assume that the strings are the same as source unless otherwise indicated, where the indication is the presence of a "translation". I think most translators would proceed component by component (e.g. MediaWiki extension by MediaWiki extension) and then check the "new additions" group for updates. If this sort of workflow doesn't work for you, please describe what a workflow should look like to satisfy your needs and maybe the developers will find a solution.