The relevant provision is Sch4 UC Regs 2013, pointed at by reg 1 DFA Rrgs 2001:
‘ “relevant award of universal credit” means an award of universal credit the calculation of which includes an amount under section 11 of the Welfare Reform Act 2012, calculated in accordance with Schedule 4 to the Universal Credit Regulations, or would include such an amount but for paragraph 3(e) of Schedule 1 to those Regulations; ‘
Sch4 UC Regs is entitled ‘Housing costs for renters’ https://www.legislation.gov.uk/uksi/2013/376/schedule/4
Sch5 on the other hand covers housing costs for owner-occupiers, so in your case the housing element is not under Sch4 and therefore doesn’t engage the DFA Regs. On top of that UC Regs 25(2) specifies that payments in respect of accommodation must be ‘within para 2 Sch1 (“rent payments”)’.
As an aside, I think Sch5 only applies to service charges (ground rent etc) rather than mortgage interest because of reg 26(3).