Obviously at a London Terminus it is different, but not everyone boards there. Even when I was at Paddington one night at around 23:30 there were no staff to be seen. All of the local services were self dispatched, this may well have changed since. Waterloo and Victoria are the same, with Victoria having staff at Gatwick Express and then maybe two members of staff dispatching trains. 8pm may have been a bit early, certainly 10/11pm seems to be when staff numbers really start to dwindle down. Gateline staff tend to disappear as soon as the peaks are over, in fact Victoria is the only London station where I can recall the barriers still being manned at 10pm, every other station has had them open with the exception of the Underground!
On board staff may be able to do something, but most will just say 'Go find a member of station staff' and if they do plan on helping then there is a high chance they will say that someone will be sent over to help you out, but that person will never get there. Obviously not all staff are like that, but quite a few are.