> Would the former British colonies be doing better or worse had they never been colonised?

It's not necessarily correct to assume that they failed because they were previously colonised by the British. Should we have instead allowed other powers to colonise them? That is the only realisitic counterfactual here. Can we truly say the world would have been better off if the British had not colonised various countries, for example when it came to waging the Second World War?

I tend to agree.

Today, the UK's Navy for example is so diminished due to cuts by successive governments, that it would struggle to defend even Britain's shores.

See for example https://www.youtube.com/watch?v=zBe_36XfZb4

UK military capability is not what perhaps persists in some people's imaginations, even compared to say 20 years ago.

Britain does need to increase military spending.