In general I would say it should be possible to estimate the date from a visual inspection of a piece to within 20 years in most cases. If you have density charts for the major British manufacturers, you can kick out certain regions in that date range. If it's not British you are a bit stuck. I'm putting together a book or two on Manchester glass so I have several hundred readings from the area, and a number from outside the area, and a few overseas. Barrie Skelcher did a lot of work in this area in his uranium books.
As regards this bottle, somebody has a similar one, but earlier, on ebay at the moment. They've called it a late Georgian or early Victorian scent bottle. I've also seen similar shapes called a pepper bottle, so it could be a condiment.