Reg 5 of the UC (Transitional Provisions) Regulations 2014 excludes people who are entitled to UC from being entitled to HB and Reg 6 of those regs exclude a person who is entitled to UC from claiming HB. Both regs include exceptions but there is no reference to housing costs in the exclusions/exceptions.
Reg 8 provides for termination of the legacy benefits when a person claims/is treated as claiming UC and from what date.
http://www.legislation.gov.uk/uksi/2014/1230/contents
The Tax Credit amendment is in this SI http://www.legislation.gov.uk/uksi/2017/376/contents/made – it still has the reference to 31 October 2018 (but that should change following the changes to the rollout timetable).
I think this all goes back to the WRA 2012 and comes into force as and when the various commencement orders are laid.
http://www.legislation.gov.uk/ukpga/2012/5/contents
Hope that's what you were looking for/thinking of?
