If the house is high value and if the estate is over the nil rate threshold it might still be worth paying for a red book valuation for reasons of cashflow and certainty. For an estate that is below the nil rate band and a house worth £150,000, probably not worth doing. For a higher value estate and a house worth £1 million, it is almost certainly worth doing.
If the average of the EA valuations is too high by £10,000, then an extra £4,000 of IHT would be due within 6 months of the date of death. if it is too high by £100,000 then an extra £40,000 would be due. Even if that ultimately can get refunded, there is a need to come up with the cash in the meantime, unless there is high confidence that the achievable selling price will be lower.
The cost of the redbook valuation might be £1k, and that money is lost, but that might be a better deal than having to pay a higher amount of IHT upfront.
Furthermore, if the house gets sold a few years later at a somewhat higher price than the redbook valuation, that redbook valuation may allow the further uplift in value to get charged at lower CGT rates instead of having a backdated increase to the IHT bill with interest charged additionally on top of that extra IHT amount. With Estate Agency valuations, there may be a higher risk that HMRC will challenge the valuation used for IHT purposes.
Caveat for @IHTProbate this is certainly not advice - I have no qualifications in this area - I just just have a certain (some would say unhealthy) level of curiosity about matters related to estate planning and IHT, and that prompted me to think about your scenario and to discuss it with ChatGPT. But, depending on the sums at stake, it may be worth checking with your own advisors and/or reading other posters who will certainly take delight in tearing to shreds the many flaws they are likely to find in my reasoning.