You cannot sell the land unless you are the owner or the person entitled to become the registered owner, and in the latter case you prove that it is so. Even where you prove it is so in the latter case, some solicitors would not accept it until you become registered, so might allow their client to sign a contract but subject to your registration completing before completion. Some solicitors might accept an undertaking on closing to assist if your registration did not complete.
The Executor could sell though, without you becoming registered, but then you would not be involved.
In relation to tax, again it depends. Is it possible to revalue on the date of the grant? If not, you may be subject to CGT on the difference between the valuation and the sale price. If you were already on the CAT threshold or subject to CAT then it makes no difference, but if you had ample threshold for CAT to play around with, then not so good.