I am an Expedition Medic (have been for many years), First Aid Trainer (up to expedition level), SAR Medic / Tracker / Trainer and a H&S consultant (amongst far too many other things). I have had to give first aid in remote parts of both the UK and abroad - as well as in more populated areas.
It very much depends on your situation.
If you are a man in the street in the UK, you dont technically have to do anything to help.
If you are at work, it will depend on your contract of employment, as you may be in breach of it if you do not help someone when you may be required to do so. If your employer has paid for your first aid course, you may well have a legal duty to help.
If you are responsible for a group - e.g. if you have agreed to act as a medic on expedition, you will have to get involved.
As with all, you must not put your self in danger - remembering that residual risk is always there. It will always be a personal call, so you have to justify your actions (or omissions) to yourself first and be prepared to answer questions later.