A formal legal right of access across another person's land is established through a process called an easement. This is where one landowner grants a right to another landowner, perhaps on payment of a sum of money.
The land you are buying clearly doesn't have this.
Another way to establish a right of access is by using the Prescription Act 0f 1832. This say that if a landowner uses another's land as an access, not in secret, not with permission, for at least 20 years, he may be able to acquire permanent right of access.
The only person that can claim 20 years continuous use is your seller - if he's been living there that long. If he has, he could sign a Statutory Declaration (sworn statement) that your solicitor could then register with the Land Registry. The LR would then enter a comment on your Land title saying ""Notice entered in pursuance of rule 254 of the Land Registration Rules 1925 on {date} that the registered proprietor claims that the land in this title has the benefit of a right of way over the {description} leading to {name of public highway on the other side}. It is my guess that your seller hasn't been there 20 years.
The purpose of the insurance policy is that in the unlikely event that the owner of the unregistered land turns up and tries to prevent the access or demand a price for an easement, the policy coughs up.
It will not affect the value of the property.