I have a spare bedroom measuring 3m x 4m which can contain a wide variety of stuff. It is safe,secure and the space has a constant temperature. The flat is extremely safe and your items would be secure here.
The space is part of a flat that is on the fourth floor and can be easily accessed via a lift or stairs if preferred. I will need to be around to let the guest in.
There is a bed in the room but that can be disassembeled to create more space if needed etc.