I am not sure what the CGT issue is. We are not told that the interests in the residuary estate are contingent. Only then would Crowe v Appleby be relevant. Even where it is, HMRC accept that the exercise of an available power of appropriation by the trustees will itself cause absolute entitlement: CG37530.If there is no subsisting contingency that event will have already occurred. You cannot use the power to pre-empt absolute entitlement if it has occurred already.
I am not sure however that this is what is meant here. The legatees may have absolute shares in residue apart from the legacies. If so the administration period would still be in operation and the land would not have even become settled property as it is not yet held on trust within s68 TCGA. The concept of absolute entitlement would be a red herring for the time being.
If the PRs sell the gain will be theirs and I suspect that they prefer to allow the legatees to take under s62 who can then realise the gain for themselves on a later joint disposal. Only the legacies prevent that as the PRs must prudently keep control of the land and any sale proceeds in order to pay them.
As Malcolm suggests the PRs could appropriate the entire asset but they would surely need either an undertaking that the legacies would be paid with an indemnity against default or an agreement that the PRs be irrevocably authorised to sell the land on their behalf and pay the legacies from the proceeds. I am not sure what this is in equity, whether a lien or charge of the PRs or just an estoppel against the legatees, though it must surely be enforceable, but a conveyancer will need to advise if any restriction etc is necessary. The PRs need consent to use the AEA s.41 power because any express power dispensing with consent is most unlikely to allow them to unilaterally impose the above arrangement (and hopefully the Will does not expressly exclude it altogether!).
No SDLT is the better view provided there is no equality money paid as between beneficiaries or to the PRs (convoluted combination of FA 2003 Sch 3 para 3A and Sch 4 para 1 reinforced by an analogy with para 16 but unhelpfully not specifically clarified at SDLTM00570).