A new, bespoke Purdey, ideally in a smaller bore, is the only gun currently made that will retain its value. Other, new, London best guns will follow closely such as H & H, and Boss. Any other UK or new Italian or Spanish gun will lose a considerable amount of value after it is purchased with the possible exception of Fabbri

Regarding used guns as investments is rather unwise, even for the best London guns. If you buy one, buy it to shoot because it will have very limited liquidity and you will most likely lose some amount of money unless you hold it for a period of time. I would look for a "best" gun made before 1939 and in the most original condition you can find. Prices for these London guns seem to be better in the UK than in the US.