If property is unregistered, ownership is not guaranteed by the state. The title can only be proved by a copy of the title deeds, and your solicitor will check back the property’s documentation over at least 15 years to certify it. With unregistered property, disputes over title are not uncommon.
When you buy unregistered property, it must now be registered for the first time with the Land Registry. This will take some time so the buying process will take longer than if you are buying registered property. Your solicitor/conveyancer’s fees will probably also be higher.