IFM21005 advises that "A Real Estate Investment Trust (REIT) is a vehicle that allows an investor to obtain broadly similar returns from their investment, as they would have, had they invested directly in property.
In the hands of the shareholder, property income distributions (PID) are taxable as profits of a UK property rental business2.
The PID is received net of withholding tax, unless it is to a recipient entitled to gross payment"
SAIM5330 advises that REITs are generally taxed as profits of a UK property business, with 20% tax deducted.
The income would be declared on SA106, page F2 "Income from overseas sources".
You can read the guidance here IFM21005 - Real Estate Investment Trusts : Background: Introduction