Disregarded income is savings and investment income from UK bank and building society interest, National Savings and Investments and dividends from UK companies.
If you were a UK resident, then these sources of UK income would be taxable.
As you are not resident, they are not taxable, but any tax deducted from them is included in the tax calculations and may not be refundable. Two calculations are required:
The first is with all the income that is taxable in the UK and
The second also include the disregarded income
The tax payable is the lower of the two calculations.
Any tax deducted from the disregarded income is set against the lowest liability and only if there is a surplus will it be refunded.
Have a look at Non-residents savings and investment income (Self Assessment helpsheet HS300)
HS304 Non-residents — relief under double taxation agreements (2020) would allow you to claim back tax paid in the UK on interest from bank and building societies, royalties, most work pensions and annuities.