It is very unlikely that someone in the UK who acted in good faith when trying to help another person would be held legally liable for an adverse outcome. No such action has ever been brought against someone who performed CPR and, in general, the courts in the UK look favourably on those who go to the assistance of others.
Resuscitation Council UK has published detailed guidance on the legal status of those who attempt resuscitation. This provides answers to most of the commonly asked questions on the subject.