The Chloride product suffered group bar corrosion too, I dismantled many of these batteries from the mid 80's until the mid 20's when BT changed its sole supplier of VRSLA's to Yuasa, who made thin plate pure lead VRSLA's.
The high temperatures in the exchanges resulted from decisions made in the 70's to change from wet cells to VRSLA's and house them with the switch and transmission equipment rather than in their own dedicated battery rooms and to cool the equipment areas using fresh air cooling for up 85% of the year (the Air Handling Units contained a small compressor refrigeration unit as well as fans). This was based on the outside temperature only being 35 degrees C or more once in a hundred years at the time of the decision, but by the mid 2000's it had became something that occurred most summers in the UK. As you say, the increasing equipment room temperatures resulted in the installation of full time air conditioning for the hotter exchanges. I have seen temperatures in excess of 50 degrees C in exchanges where the high summer ambient temperatures coincided with a mains failure.
I witnessed some "System X" tests at Plessey in the very early 80's where the switch and transmission equipment was still working at 70 degrees C, however the rate of change of temperature had to be closely managed to get the equipment up to that temperature and back down to normal again, and the batteries were useless after being subjected to that temperature for a few hours.