I think Stratford (East London) was pretty bad for air pollution too - wasn't it the largest steam MPD in the country in its heyday?
I don't know, but would think so. It had something like 450 - 500 locos before electrification began to eat into the allocation. A staggering number. And given the prevailing winds, it would also get a little of the junk from depots to the west (Kentish Town, Kings X, Camden, Neasden).
The Clean Air Act was passed in 1956, after the Modernisation Plan had been published, and was mainly a response to the London smog of December 1952.
The Eastern Region gave the London area priority for modernisation, both on the ex GE and ex GN lines, but I've never found a definitive link between that and the Clean Air Act of 1956. What is much clearer is the difficulty of recruiting staff to work on steam traction when there were other options in a buoyant London labour market, particularly Ford at Dagenham.
Even without the labour shortages and air pollution, it would make sense to eliminate steam from the south end of the GN first, if only to save a few WDs on mineral trains carrying coal all the way to Kings X and Hornsey every day.
However, strictly speaking, all this is off topic - the OP wasn't advocating keeping massive shed loads of steam, just a limited number to extract a better economical result from the original investment.
Having written that - it is an appropriate thread in the sense that it is August 3 - 55 years to the day of the last standard-gauge scheduled steam-hauled service on BR.
God Bless Ernie Hayes and his fireman!