Also, if you look at the original article, you can see the confidence intervals around the figures
Lancet article
So for the UK, the confidence interval is 4-6 deaths per 1,000 births and for Western Europe as a whole it's 3.3-4.7. In other words, the confidence intervals overlap so the differences in child mortality between the UK and the rest of Western Europe are not statistically significant and could be attributed to random error.
I think it's a storm in a teacup and clearly the figures are being misused for political purposes. Obviously, any child death is a tragedy, especially preventable ones, but the UK is really not doing badly at all comparatively.