How a chain of leases works
A freeholder grants a long headlease; the headlessee grants individual flat leases out of it. Your immediate landlord is the headlessee, and their obligations to the freeholder are set out in the headlease — obligations similar in kind to the ones your lease imposes on you.
The dependency runs downwards. Everything below the headlease exists because the headlease exists.
The risk, stated accurately
If your immediate landlord breached the headlease, the superior landlord could seek to forfeit it, and your own lease could technically go with it. The honest framing is the one the Lexsure wording below uses: a minor risk, and a frightening and costly one if it ever materialised.
There is a remedy. A leaseholder in that position can apply to the court for relief against forfeiture, and relief is commonly granted to people who have done nothing wrong. It is expensive and it is not automatic.
The practical frequency of this is very low. That is why it is insured rather than investigated at length.
What “unregistered” adds
It adds blindness. A registered headlease can be inspected: its terms, its parties, whether it has been varied, whether anything is noted against it. An unregistered one cannot, and where the freehold is unregistered too, nobody can confirm whether the headlease has been merged into it.
So the conveyancer may have a copy of the headlease and still be unable to confirm its current status. That gap is what the forfeiture indemnity policy addresses.
Ask who is paying for the policy, and keep it. Ask also whether your immediate landlord is paying the headrent and complying with the headlease, because the risk is created by their conduct rather than by yours.