On a personal note, I can remember the days when coal was still burned in large quantities in Britain. Later, when I visited East Germany, I wondered why I kept thinking of my early childhood. Then I twigged: it was the smell of coal smoke.
The French brewers have attempted to imitate the London porter, by hiring brewers from the metropolis to brew there; but however fully these have exerted their talents, they have not been able to brew a porter exactly similar in taste to the London; and the savans of the French capital have been puzzling their brains to determine the cause of this failure.
Mr. Francoeur, professor to the faculty of sciences at Paris, imagines, for what will not man imagine that the peculiar taste of London porter arises from the smoke of our sea coal fires. He says, that the peculiar smell of the smoke of sea coal is so abundantly diffused in the atmosphere of London, and especially in those manufactories which employ great fires, and in their neighbourhood; and is so powerful and adherent, that in London every article is strongly impregnated with the smell of it. English cloth, packed in the metropolis, carries the smell into foreign countries; and the clothes of Londoners retain the smell for a fortnight after they have left their homes. Hence, he says, there is no wonder that the porter should taste of this smoke, especially considering how long the porter is exposed to the smoky atmosphere, with a large surface in coolers of the brewhouses, whose steam engines emit such dense clouds of smoke.
"American Mechanics' Magazine", Saturday August 6th 1825, pp 62-63.
[By "sea coal" they mean what I would call coal. The lumps of black stuff that you set fire to.]
There could be many explanations why they couldn't brew Porter that tasted like the London variety. Raw ingredients, brewery equipment, water chemistry. Without any more detailed description of the difference in flavour, guess is about all we can do.