I take it the beneficiary in prison has separate legal advisers.
I suggest the beneficiary should be informed that if he wishes to disclaim it is an “all or nothing” situation, and that he cannot direct where the inheritance should go if he does disclaim. He should be directed to his own legal advisers to ascertain if it affects his legal aid status. I can see no justification for the estate bearing the cost of identifying that position.
Whilst it is trite law that a beneficiary cannot be forced to accept a gift against their will, you need to be satisfied that it is “their will” and that they are not under a misapprehension as to the effect of their refusal.
Whilst it is arguable that any form of disclaimer is valid, counsel usually advises that a disclaimer is made by deed, to reflect the seriousness of the action and so that the personal representative can be satisfied the disclaiming beneficiary has been advised on the consequences of refusing the gift. A mere letter can be revoked if the original beneficiary changes their mind, especially if they decide they had not understood the consequences of a disclaimer.