Where a Universal Credit claimant is getting Housing Benefit to help them pay the rent for their specified accommodation whilst the HB they receive is classed as 'welfare' when applying the Cap to a Universal Credit award, UC Reg 80 para 2a states that the amount to take into account is always nil.
However a claimant may still be affected by the Cap if their total 'welfare' excluding Housing Benefit exceeds the Cap limit.
More on the Benefit Cap here.