I would first look at how the infant deaths are calculated. I’ll bet the calculations themselves are different, favoring the Brits in this case.
The Brits do count infant mortality rates different than the US along with the other mortality rates. They count babies born later than the US. The Brits other mortality rates are not as accurate either. They have higher death rates from all kinds of cancer, because they use cheaper drugs and older ones that are not as effective. They also pull the plug on people after a set number of days (regardless of age) and with hold food and water and give pain medication until they die. They list this as natural causes. (not hardly) So their rates are calculated differently. No one tells you that fact.
It’s not the infant deaths that skew the figures, it’s the live birth rate. We use neo-natal care to keep babies alive and count any baby that takes even a single breath as a live birth. Some European countries actually have minimum weight and length rules on whether a baby is worth saving, whether it is born alive or not.
Gun deaths, gang activity, etc. , also enters into the mix.