The lease states that sub-letting is OK if landlord informed which is fine. But can it be done as an informal agreement between the 2 people without involving the landlord?
In most cases I would say no.
Also would any agreement be legally binding should there be an issue, i.e. a roof repair or similar, so would it remove the liability for the second person?
A scenario, you make an informal agreement. The roof falls in and is going to cost a lot. The other party decides to forget about the handshake and refers to the original legal agreement. You would still be liable.
Also, if you are still on the lease and the guy who agreed to take it over informally went bust, the landlord will be coming to you for the rent.
I would say that for piece of mind, limiting the chance of any surprises in the future, the goodwill of the landlord etc, it is worth the cost of sorting things out legally.