So, those paying 4.5/365 are illegal or just morally a bit rubbish?
Generally speaking, just morally rubbish.
In the last twenty years or so employment contracts in the UK have become vague regarding working days/hours etc. As an example, the standard for our normal office based staff is along the lines of “a standard 35 hours per week or as required in special circumstances” and will also state “Normal office hours are Monday to Friday, 9am to 5pm, with 1 hour for lunch, or as required in special circumstances”.
It’s a kind of catch all so, for instance, if there are adverse weather conditions or travel issues these can be changed to the employee’s benefit or, more likely, workloads/time pressures mean that the employer can insist on extra from their staff.
Not quite down there with zero hours but it does give the employer the whip hand, so to speak.