The problem with apparent minor injuries is that they may be a symptom of something not so minor, which has already happened (e.g. earlier blow to head etc), or something which is about to happen. Blood coming out of anyone unexpectedly isn't right. Most of the time these things remain minor and are easily treated. A first aider should remain with a casualty where possible until their treatment is effective, they're obviously getting better or you hand over to someone else. Another point is that I quite often found myself treating not only the original casualty, but other people (frends, relatives etc) who had decided suddenly that they didn't like the sight of blood and were now also feeling a bit wobbly.
West Lakes, the Good Samaritan laws don't exist in the UK, but as the link explains, you would more than likely be able to put up a defence against possible claim against you for battery or negligence, thanks to some wording regarding what is a reasonable course of action for a "rescuer" (!). So you'd be relying on being able to
successfully defend your actions rather than there being any law saying you can treat people without fear of a claim being made against you. Although
There are likely, however, to be limitations on the application of these defences when procedures are carried out by non-professional rescuers and the less well trained the rescuer the harder it may become to justify either defence. It becomes more difficult, for example, to argue that an unconscious person has given implied consent to a (relatively) untrained person performing what is in effect a medical procedure, notwithstanding that the procedure may be straightforward, automated and mechanical. Similarly, it may be harder to argue that treatment by a lay person is in the best interests of the casualty. Only if the action taken by a lay person is, under all the circumstances, ‘reasonable’ and in the best interests of the assisted person, is there likely to be a defence.
In this case though, I think it would be pretty difficult to defend not doing anything at all when it is part and parcel of the job.