If the estate is definitely going to be subject to IHT then there is no point at all in paying for a professional valuation.
The basic objective is to put as low a valuation of the house as you can get away with, thereby minimising the amount of IHT you have to pay.
For most ordinary houses an online valuation from Zoopla or similar will do - just knock, say, 10% off, and if asked you can always say you did so as it needs work doing.
When you submit the IHT405 giving details of the house and its value it will be sent to the District Valuer. The vast majority are accepted without query, provided the valuation doesn't appear to be seriously out of line. If the DV thinks it has been significantly undervalued they will say so, and you can then negotiate with them to agree a value.
Only if and when you get to that stage is it worth paying for a RICS surveyor, as they can hopefully save you more than the cost of employing them.
If the house is to be sold it doesn't really matter what value goes on the IHT405 as it will be adjusted following completion of the sale. Finally, it's always better to pay CGT than IHT as the rates are much lower.