Do UK universities have a special obligation to advertise jobs externally?
I think you are misunderstanding how academic job searches work in the UK. All jobs, and I believe this is not specific to universities, must be advertised internally first to individuals in the "redundancy pool". If, and only if, there is no one in the redundancy pool that can meet the minimum requirements with 6 months of training can the job be advertised externally. The redundency pool is available to people, I think, from 3 months before their contract ends until 3 months afterwards.

Current employees not in the redundancy pool, can get prompted within the same job family without having to advertise the job at all. At my university there is no system for moves, lateral or upwards, across job families. That said, if you really want someone, you can regrade and reclassify them.