Plumber

That is a interesting question.  If losses in any year in which she is a non-taxpayer are less than the gross p2p income for that year it would be perfectly fair for her to get no relief.  She'd just have an amount of net income that is tax free in her hands.

I think the legislation achieves that, because (paraphrasing for simplicity) s412(C)(1) permits a loss to be carried forward only if, in the year that the losses arose, the lender has no, or insufficient, p2p income from which to deduct the loss.  I think this test is applied without regard to whether the lender is taxable on the income.  The actual words of the legislation are:

412C Claims for additional relief: carry-forward relief
(1) A person (“L”) may make a claim for relief under this section if—
(a) L is entitled to relief under section 412A in respect of any outstanding amount of the principal of a loan (“the relevant loan”), but
(b) in the tax year in relation to which L is entitled to that relief (“the relevant year”)—
(i) L has no income of the kind mentioned in section 412A(4) or section 412B(3) from which to deduct the outstanding amount, or
(ii) L has insufficient income of that kind to enable the outstanding amount to be deducted in full under those sections.

If in any year her losses exceed her p2p income then she will be able to carry forward the excess for up to four tax years [s412C(2)], but the carry forward rules work so that the loss must be claimed as soon as possible.  If, in the subsequent years before she becomes a taxpayer, there is enough net p2p income to absorb the carried forward loss, there will be nothing left to claim by the time that she becomes a taxpayer.  Below is the text of s412D which explains all this:

412D How carry-forward relief works
(1) This subsection explains how deductions are to be made under section 412C. The amount to be deducted at any step is limited in accordance with
section 25(4) and (5).
Step 1 Deduct the outstanding amount or (in a case within section 412C(1)(b)(ii)) the part of the outstanding amount not capable of being deducted under sections 412A  and 412B from the lending income for the first tax year following the relevant year.
Step 2 Deduct from the lending income for the second tax year following the relevant year any part of the outstanding amount not previously deducted.
Step 3 Apply Step 2 in relation to the lending income for the third and fourth tax years following the relevant year, stopping if all of the outstanding amount is deducted.
(2) In this section—
“lending income” means income of a kind mentioned in section 412C(3);
“relevant year” has the meaning given by section 412C(1)(b).

Finally, I don't think there is any way that you could claim the loss yourself.  It might be worth considering whether you, as a self-employed person, could pay your wife a salary to get her to a taxpaying position.  But I have no knowledge or experience of what you might be able to get away with do in this regard