They have said their solicitors have said they can’t sell it to a relative either and have been asking why there has now been a planning application submitted.
This is legal, not tax, but
a) it's wrong that you cannot sell to a relative. You can almost always sell to a relative. The problem with selling to a relative is establishing whether the property was sold at a value different to market value. But if you've matched the highest offer on the property from an unconnected third party then it's hard for anybody, including HMRC, to argue that it's not a transaction at market value!
b) Anybody can submit a planning application for anything! I could submit a planning application to demolish the houses of parliament and build an amusement park on the site. It wouldn't get granted, obviously, but you don't need to have any interest in the land to submit a planning application. I don't know how common it is to submit planning applications for land you don't own but the obvious reason would be that you're only prepared to buy it if the application is granted.
(I have no idea how this affects market value, clearly land with a pending application is different from land with a granted application, but in the HMRC case the land value at death is "without a granted application" so the value is that which an unconnected person would pay not knowing if the permission will be granted)
Note that the property can be sold two different ways - I don't think it affects the IHT. Either the executors can sell to you or the beneficiaries of the will can sell to you after the estate is distributed. I don't know exactly how it works but in the former, the proceeds of sale can be used to pay the IHT bill while in the latter, the IHT bill will have to be paid first. (But you could lend the money to the estate to pay the bill).
Note that there is a third way to get the property to you, and that's to vary the will (I think this can be done even absent a will). The beneficiaries have to agree to this - but if everyone was in agreement that this was what your uncle intended then it might be possible.
Additionally, if your uncle had a (living) spouse, then it's possible to vary the will and avoid all the IHT completely.