To be fair, it's only relatively recently that pollution from diesels has become a significant issue in people's minds. Certainly, back in the 50s and 60s, I doubt many people thought about the issue - and if they did then, well diesels were a lot less polluting than the steam locos they would have replaced. If my understanding of history is correct, to the extent that pollution in cities was a political issue, it would have been smog etc. caused by burning coal. Next to that diesel pollution would have been trivial. So you can't really blame people (or politicians) at the time for not being aware of the ecological consequences of diesel trains.
In fact, come to think of it, I'm not too sure what the reasons for electrifying would have been in the 1950s and 60s? Faster acceleration and better performance up hills I guess. Was there anything else at the time?
Quite so.
I was at college in London in the early 1960s and experienced the last great smog in the winter of 1962/3, see
here for a description. The stink of sulphur was like living at the gates of Hades and losing your way when trying to cross a road because you couldn't see the other side was scary - I still remember it.
Diesels were clean compared to all those millions of chimneys burning coal and all those steam locomotives in London - of BR's 15,000-odd steam locomotives five or six thousand were in or near London.
People faffing today about diesel pollution - and vehicles have catalytic exhaust treatment now - are wimps!
The advantages of electric traction for suburban services were clear - rapid acceleration compared to six-coupled steam locomotives, no need to replace the locomotive at each end of the journey and reduction in smoke and fumes in the roofed-over terminal stations. For longer distance journeys 2,000 to 2,500bhp diesel locomotives were a considerable improvement over the average steam express passenger locomotives - not all of them were A4s, Duchesses or Kings, most were Black 5s, or Halls, B1s or N15s - and the diesel maintenance costs were a lot lower. At the time longer distance traffic was being lost to the airlines - the Motorway network was far from completion - and it was by no means clear that faster trains would make a significant difference to the overall number of passengers carried. Also electrification required a massive capital investment several years before any benefit was apparent - the whole route had to be electrified before the passenger noticed much difference. BR used to change traction at Crewe and then Nuneaton as the West Coast electrification slowly worked its way South so the time gained on the electric stretch was lost in the traction change. Buying diesels meant the improvement was immediate - even though the Deltics were a year late in delivery. (Sounds familiar...?)