The last bus of the day not appearing will lead to people being stranded as there is no way to get home afterwards, especially when long distance is involved such as in the case of interurban buses.
It's unlikely that one will have the mean, for example, a few dozen pounds, to pay for a taxi for a long distance, or if one can even see a taxi at all deep in the rural area waiting for the last interurban bus home.
Well you need to have the means, or have a plan to call a friend or relative to come and fetch you, or risk a few miserable hours waiting overnight or walking. Most areas will have a taxi service you can call on, even if they cannot provide immediate service. This is no new thing. However, it doesn't happen that often.
Not get yourself into that situation in the first place by ensuring you have enough spares on duty until the last bus leaves service, or fork out for taxi or another company to cover the run.
Sadly, these kind of costs can no longer be borne by the bus industry (if they have ever been since the 1960s?) - the bus company will usually refund a taxi in this circumstance, but you'll be on your own to arrange it, pay up front and claim it back.
Enforcement by whom? Outside of London, unless it’s one of the very rare exceptions, bus services aren’t specified by a public authority but run on a commercial basis.
Phoning the police because your bus hasn’t turned up sounds like one of the examples the UK police give of people wasting the time of 999.
The regulatory authority have the aim that 95% of bus services must run no more than 5 minutes late or 1 minute early. The cancellation of a last bus may well fall within the 5% that are permitted to be non-compliant, and anyway punitive action is only taken in extreme cases, for various reasons.
Unfavourable comparisons with the railways are all very well, but the bus industry operates in a completely different regulatory, and particularly funding, framework.