For me, over-signing somebody's worksheets / engineer's log book entries is you verifying that the person has carried out what they have recorded as carrying out and that the person is competent in the task they have recorded.
IMHO, the best person to do the over-signing is the licensed engineer who worked with that person and certified for the work.
The UK CAA Personnel Licensing Department definitely scrutinise worksheets / engineer's log book for the required jobs and experience but whether they check the licence of the people over-signing entries, I don't know. It may be that PLD expect the people signing block 13 (Assessor's Certification and Evaluation) and block 14 (Referee - normally the Quality Manager) to have checked the validity of the worksheets / engineer's log book entries.