Agricultural property relief should be due on the land, buildings and cottages if these are used for agricultural purposes at the time of death, and have been so used for the previous two years. To the extent that there is any development value, this would be covered by business property relief, assuming he is farming the land. These reliefs should exempt all of the assets from IHT. The farmhouse may not be fully covered by agricultural relief - it depends on whether its 'agricultural value' is less than its 'market value' the ag value being what it would fetch if it were subject to an agricultural covenant. But if some of the farmhouse is taxable, you still have the two nil rate bands to hopefully cover any taxable value.
It does not matter (as far as IHT is concerned) what happens to the farm after your uncle has died. The IHT is just based on the position at the time of death.