IHTM10463 - Checking the declaration (box 119): estimated values
The personal representatives may include estimated values in the IHT400, as long as they have made the fullest enquiries that are reasonable practicable in the circumstances to establish the correct open market value. The personal representatives should list in box 119 all the boxes in the IHT400 and supplementary Schedules where estimated values have been used. The declaration places the onus on taxpayers to tell us the exact values as soon as they are established.
You should generally assume that all values returned are open market values unless they are listed in box 119.
There are instructions on dealing with estimated values (IHTM09035) in section 9.