value at date of death
When it comes to valuing property for IHT purposes, the value that must be established is referred to as the 'open market value' which is the value that the property would reasonably be expected to achieve if sold on the open market at the date of death (Section 160 of the Inheritance Tax Act 1984).
If there is a large share holding then buy ( clearly you dont) buy a copy of The Times or FT on the day of death
If the valuation is 200k and the lucky heirs sell for £300k three years later then they pay CGT on the gain
( if there is a loss, then they can claim back some IHT - not often used I can tell you. ie it is not a claimable CGT loss)
Taxation of estates is not rocket science
https://www.gov.uk/hmrc-internal-manuals/inheritance-tax-manual
This is TERRIBLY boring but clear and covers just about everything - inclluding huge swathes you can skip as it doesnt concern you