Are you saying the UK outsourced all it's heavy industry off-shore to cut-down upon emission locally?

I doubt they're saying that the UK outsourced heavy-industry specifically to reduce on local emissions, but rather that one of the secondary effects of outsourcing heavy industry was to outsource pollution.

I shared those doubts, hence asked for some clarity to avoid any subtext.

Manufacturing is and always has been a complicated one. Until there is some global emissions tax upon products that factor in full material to consumer aspects from production and shipping emission - things will never change and the ability to have a local product cost 2x more than some imported one that has 10x the emission, is still going to play out until there is a true emission tax (not just carbon). Hence the whole emission/environment and humanity impacts of industry has no true global standard and the variance of those standards is a difference that we all end up paying the difference, albeit indirectly.

Could be abused as a non tariff barrier see the chicken tax on light trucks and other ways the us protects its local car industry.

Yet if the whole environmental factor of shipping/transportation and production emission standards are factored in - those cheap imports would become less cheap and negate the need to allow some local production to almost die before noticing that it's unfairly suffering due to it having to comply with a higher level of environmental and worker standards compared to some import.

Hence you get workarounds happening like the example you mention about chicken tax - https://en.wikipedia.org/wiki/Chicken_tax workaround that can play both ways, yet if there was a base-level standard and measure to balance things out, then such loopholes become less harder to happen, or needed. Alas the only common standards are weights/measures and currency, everything else does vary.

As an aside, I dare say that the history of importing chickens into Europe is more fascinating than most imagined, and one that still plays on even to this day.

It's just a side effect, though I'm certain that such was a factor in the high level decision making. "We tore the heart out of our communities, for better local air quality. Also we really stuck it to all those foreign countries we polluted."

Also manufacturing to China, no?