Many London buses are similarly adorned and unfortunately many passengers obey those instructions.
Indeed. Before the advent of London's "Routemasters" the principle double deck class was the RT family (RT/RTL/RTW):
www.countrybus.org
These open platform buses had no heaters (apart from a very few buses used for "Green Line" service, which had a very rudimentary heater retro-fitted in the lower saloon). They were absolutely freezing. The Routemasters were scarcely much better. They had a grill at the front of the lower saloon and two small grills at the bottom of the top front windows. These dispensed a small flow of lukewarm air and were more draught producers than heaters,
Passengers don't know they're born today! They need to man up!