The specific legatee or (per Old Testament) devisee is absolutely entitled to the income from the gift subject to:
(a) the right of the PR to take it to pay debts, including tax which is a testamentary expense, in a solvent estate
The order is set out in AEA s34(3) and Sch 1.
https://www.legislation.gov.uk/ukpga/Geo5/15-16/23/schedule/FIRST
A specific gift is way down at category 6. The doctrine of marshalling allows a legatee to recoup any loss from anyone in an earlier category but they cannot recoup downwards to the specific gift. This is per Snell citing Re Wilson [1967] Ch 53. Snell is silent on income from the gift but “property” in the AEA presumably includes any income from it. Also, presumably, even if it is paid direct or “mandated”. These omissions from Snell must indicate no relevant case law (despite being described by my Director of Studies the late John Tiley as “a real book”, for so it is).
Category 8 allows a Will to establish an overriding different order. I have never seen one that did, not even in my worst nightmares about Wills and the SRA, from both of which monsters I am now liberated.
(b) the PRs can be assessed at basic rate on income they receive but are not entitled to (ITTOIA 2005 in each source of income provision).
HMRC TSEM goes on at length about IIP income, mandated or not, but a specific gift is not an IIP as not settled at all. See TSEM3761.
Specific gifts and income from them is dealt with cursorily and thus not helpfully at 7490 citing Hawley.
In my view PRs act as a nominee receiving income belonging to another, who must file his own returns and claim credit for any tax on it paid by the PR. It should not go in the PR’s return. It is not estate income for tax, even though within (a) above. Category 7 is not even an asset of the estate. How this works with online SA returns heaven knows. Probably goes in the white space equivalent. Phone HMRC if one has a couple of hours to spare and a client who is impervious to costs! Not MTD then.
An interesting previous discussion of mandated IIP income is at
https://trustsdiscussionforum.co.uk/t/when-is-income-mandated/3854