There are thousands of cabs without air con. Here in Scotland only the 158,170 ,334 and 380 have it. The rest - 314,318,320,156 don't and never will. The vast majority of BR units don't have it.
Aren't the 314 and 318 soon heading to the scrappers, to be replaced by A/C units?
Either way, the existence of non-fitted stock is no excuse for having failed A/C in units that have been designed for A/C cooling.
It is there, it is supposed to be working, if it does not work then it shouldn't be in service.
If the staff (drivers, guards and catering crew) refused to work the units with broken A/c the TOCs would soon be forced to sort the issues.
The only issue really is that they don't consider it important enough to be working on 100% of stock released for service as they do with dozens of other systems on the trains.
Plenty of parts on trains have faults and failings. A/C happens to be relatively brittle on moving vehicles, if it isn't given the correct maintenance and monitoring it will fail, it isn't unigue in this but it does seem to be the biggest system onboard whose failure is considered acceptable to ignore for days/weeks/months.
I know that because it has gone on for so long that it is more difficult for staff to raise the bar and refuse to work failed units but as far as I can see that is the only thing that will change the status-quo.