Old Tom gin is the forerunner to the modern Londron Dry style of gin. Before London Dry came along in the late 1800's the world basically drank two forms of gin: Old Tom and Genever (Dutch gin). The former is a sweetened form of what we commonly drink today with many of the same botanicals and flavorings.
A few Old Tom gins have come back on the market after a long slumber for this cocktail ingredient category. I have sampled some and I found one (Hayman's) to taste very similar to a sweetened London Dry such as Beefeater.
Now I have a booze bottle "budget" for bottle space and how many bottles I really want to have around potentially having been opened and now slowing oxidizing. So far buying a bottle of Old Tom has not made the cut. But because Hayman's is so similar to Beefeater, I just make my Old Tom by sweetening Beefeater as needed:
Combine and shake vigorously to dissolve the sugar. That's it. This fits nicely in a 375 mL bottle. I use sugar instead of syrup so as not to dilute the strength of the gin.
Give this a try and let us know what you think.