TL;DR Follow this link to book a first aid course. These are definitely lessons to learn before you need them.
After a stranger collapsed in my arms recently and stopped breathing, I finally attended a St. John Ambulance Essential First Aid course.
Our trainer Liz was phenomenal. Four hours of intense, detailed, brilliantly explained guidance on dealing with all manner of situations. We had many questions and every single one was answered comprehensively.
I learned that over 90% of people in Germany hold a first aid certificate (it is mandatory and tied to their driver's license), whereas that number in the UK is less than 40%.
I learned that being able to give CPR to someone unresponsive and not breathing increases their chance of survival by around 20%, and that the appropriate use of a defibrilator (an AED) can increase this to nearly 75%.
For an investment of just £30 and an afternoon, I would encourage everyone to consider learning some essential skills that hopefully you’ll never need.
There is a great St. John Ambulance app, which considering we pretty much always have our phones with us, and as it is free, it seems a no-brainer to search for it in your app store, and to install it today.
Update: people asked me what happened to the lady in the end. I'm relieved to say that she more responsive when the ambulance departed with her. An amazing 999 operative calmly guided me through the steps I needed to take, and fortunately as I was in central london at the time, an emergency response vehicle arrived in about 7 minutes (perhaps the longest 7 minutes of my life). I can't imagine how things might have turned out if I was somewhere more remote, and indeed a person on the course had attended to an injured person for an hour on a remote biking trail before help arrived....