Baggage contents checks are not made by airlne staff at all. They are made by airport security personnel who do not take into account which carrier you are with, nor is this part of their responsibilities. The security rules are set by the relevant national aviation authority, to which certain airports may add their own issues.
The only exception is where the baggage is sufficiently large to be obvious to airline staff what it is; for example sport parachute rigs that have AOD devices (which deploy with a small explosive) may be checked or prohibited, because it is obvious when checking them in what they are. But these are the exception.
Many airlines prohibit in their rules the carriage of Christmas Crackers, or allow them in small quantities only. I can assure you that large numbers of these go westbound across the North Atlantic to the USA (where they are unknown) in pax baggage on every flight in December.
Checked baggage temperature of the contents, on a short haul flight like this, does not actually vary a great deal from the ambient at point of departure, and here I would be perfectly happy to check these (provided of course you can accept the small risk of mishandled baggage on arrival, but that is a separate matter). A minority of the holds are heated like the cabin (to facilitate the carriage of animals etc), most are not, but the temperature does not fall off a lot inside the bags. All holds on jets are pressurised like the cabin is.