Precious metals
Let's go in a different direction, I believe the main reason for the secure compartment was precious metals from South Africa e.g. Gold and Diamonds.
The Comet was designed in the late '40's and entered service in the Summer of '52 on the route to Johannesburg.
It is quite reasonable to assume that in the event of a mishap the wingtip would have a better chance of staying intact and also not be the target of scavengers that would attend such mishap over Africa of the day.
This is not pie in the sky, I have also been aware of this compartment for the same period of time, it is interesting to see it's existence being highlighted after so long.