Ultimately this could end up in Court - you have to consider whether the costs of obtaining an order for sale are going to be worth it. It really depends on the actual difference between the "knock down" price and the sale price.
Furthermore, whilst the property stands empty, there are costs of insuring it and looking after it.
If the executors disagree, they can seek the directions of the Court. What the Judge will do will depend on the evidence. Both sides will undoubtedly put in evidence of valuations. But such evidence is only an opinion as to the value. The true value can only really be ascertained by offering the property on the open market and seeing if it sells. The Court could order a period of offering on the open market, with a long stop date of an auction. It could also order an auction (but remember for standard properties, auctions often result in a reduced value).
Would it be possible for the executors to agree the property be marketed for, say, 4 months and if no realistic offers are forthcoming, then it be auctioned or sold by sealed bids?
As for HMRC, the sale price can be substitued for the probate valuation. It is only if the sale is at a significant undervalue to a connected party that the District Valuer is likely to get involved.