Modeller. There's a couple of things you need to understand here if you are acting as an executor.
The assets are valued at the date of death for the purposes of IHT. That much is really simple.
However, the CGT issue with regard to the estate after the date of death is a whole different ball-game. If the underlying assets in the estate increase in value after death, then CGT may well be payable by the estate. The gain is assessed as the difference between the eventual sale price of the asset, less its value at probate. This is true of all investment assets AND the deceased former principal private residence (since exemption of tax on any CG on this asset is lost on the death of the homeowner). HOWEVER the estate is entitled to the normal CGT annual allowances before tax is payable - so the timing of assets sale by the exceutor in a rising investment market can seriously impact the CGT liability.
There is yet more complexity. If a deceased asset is not sold but TRANFERRED as part of a legacy in the will to an individual (applies to both shares, property etc.), no CG is regarded as having been made - even if the underlying asset has increased in value between the date of death and the date of actual transfer. Instead the receiver of the legacy asset is liable for any CGT due when THAT PERSON eventually disposes of the asset - possibly years later in the case of a family home.
This is a complex area, and to get it right (aka. tax efficient) one may need to take professional advice.