With English hallmarks you can tell the assay office and year from the hallmarks - ie a lion head = London and letter lower case “p” = 1915. The market, or sponsor mark is optional and there may not be one. Gold assay in the UK system is 375 or 9ct. The US and some other countries uses a “k” instead of “ct”.
The value of your fob chain is best checked via eBay.com or eBay.uk.com. Look at sold listings. You should expect to get more than melt value of the gold and slightly more than an modern chain of similiar gold weight. Mostly it is gold weight that dictates the value. There is no particular “extra value” for older gold jewelery except if the maker is Tiffany, Cartier etc