Section 2 of the Housing Act, 1988 states that a person should be considered to be homeless if:
(a) there is no accommodation available which, in the opinion of the authority, he, together with any other person who normally resides with him or who might reasonably be expected to reside with him, can reasonably occupy or remain in occupation of,
or
(b) he is living a hospital, county home, night shelter or other such institution, and is so living because he has no accommodation of the kind referred to in paragraph (a), and
(c) he cannot provide accommodation from his own resources
I presume hotels fall into the "other such institution". I recall a few months back picking up a work colleague who was over from the UK from a hotel and was surprised to see parents sending their kids off to school from the door of the hotel.
Charities refer to visible homeless (rough sleepers etc) and hidden homeless (people in hotels, B&B's etc)