The Title Register is the document that gives you the best chance of finding out the age of the site you are looking for - though you may need to do some detective work first. The Land Registry have a great blog about this worth reading, if you have a few minutes: https://hmlandregistry.blog.gov.uk/2018/01/26/how-old-is-my-house/
You can add the the official Land Registry document(s) to your Basket by clicking on the 'Register & lease schedule' button in the Information Panel, which opens when you have clicked on the red freehold outline or green leasehold pin you are interested in.
You will need a card to pay for the document, which comes directly from the Land Registry.
Once paid for, the Register will appear in your Manage Account section (top right) in the Documents Tab - click to download it. In most cases, the document is emailed to you as well.