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Right, one of the biggest issues that has been put forth for the sustainability of the British Empire is that you can only have so many white settlers. The British could simply not pump out people fast enough from Anglo-Saxon, or at least White, demographics.
So challenge is to make a unified 'British' Culture, or at least a 'British Mega-Culture' (The difference being that the former, everyone sees themselves as one culture, the latter being a strong family of cultures that benefit from unity).
Bonus points for preserving as many cultures/cultural features as you can.