Subject: Re: Any thoughts on UK real estate?
Does that 20% withholding apply to said UK REITs held in a US retirement account (Fidelity is the provider)?

My entirely uninformed guess is that yes, the withholding tax still applies. Plus, as it's an untaxed account, my uninformed guess is that there is no way to claim a tax credit for the tax withheld, unlike shares held outside a retirement account.
But check with someone who actually knows the real answer, not someone who is guessing based on extrapolating from other situations.

In some cases I have found the most reliable answer is obtained by buying a small number of shares and waiting for the first dividend to be credited. Particularly true for anything in the Brookfield family of countries because of the multi-jurisdiction spread of the source of the dividends.

Jim