MPs on the COBRA panel have to travel across the country on a bus. One of them asks if there will be a message on the side of the bus, the response being no member of the public would believe what they might read if such a message was placed there. This is a joke about the Brexit referendum of 2016 where one of the key messages was about the NHS (and how much Britain would save if they withdrew from the EU) which famously appeared on a side of a bus. And was later roundly proved to be a falsehood.