ESA and UC

My client, who is aged 59, is moving to a privately rented flat on Monday, which is outside the area of his current council.  He is currently receiving DLA for low rate care and higher rate mobility, ESA support group of
£130.90 per week plus full housing benefit. He is extremely worried about claiming in the new area, so it needs to be as simple as possible.

The housing benefit office in the new location told him that he should claim housing benefit and not UC, as he receives the severe disability premium. However, when I spoke to HB, they agreed that as there is no SDP award, it should be UC. They also said that if UC was claimed when there was still an entitlement to HB, the UC award could not be reversed. 

I am about to suggest he claims UC but if he claims UC when he could have claimed HB, presumably it could only be because he is entitled to the SDP and claimed incorrectly, so could it be reversed back to HB?

If he claims UC now, will there be gap in his housing allowance and can he claim any transitional protection?

Would appreciate any help - sorry to post on a Saturday.

The information from the 2nd council is simply incorrect. He does not qualify for SDP because he is only getting lowest rate care DLA so the SDP gateway doesn’t apply. He isn’t moving into temp/supported housing and isn’t pension age, so no other basis for a new HB claim. He will actually probably be better off as UC SA + LCWRAE is more than the ESA he currently gets.