£300 should buy you a bottle of pre-war port, but you will have to shop carefully and will probably not be able to go for any of the big names. Peter Wylie
has an interesting selection in your price range - halves of Cockburn 1935, or a bottle of Delaforce 1935 for example.
Whatever you buy, I would encourage you to call up and have a chat with the merchant to make sure you know what you are buying - what is the fill, is the cork sound, is the wine still red in colour, etc.
Best of luck in your quest.