I find myself thinking about what I would do in such an incident. It would depend on the circumstances and what I could viably do. At any rate, we all have to mentally prepare ourselves for such an eventuality. Learning first aid is a good idea as well - I have done an online CPR course and I may look into getting something more formal in future.
(I have been involved in two situations where I have had to call 999, both at the same railway station, although in those cases, I was the one making the call while others were attending the victim).
In addition, businesses need to add an 'Active Shooter' plan to their list of company policies.
The police killed the three terrorists in eight minutes from the first 999 call, but that was eight minutes in which they managed to kill seven people (so far) and injure 50. If this had been in somewhere like Rugby or Hull, they wouldn't have been there in eight minutes.
It is now time for a serious discusion on routine armament of the police. If not handguns, most certainly Taser. Because these people could strike anywhere with no warning and they need to be stopped quickly.
These people think they are doing God's will when in fact they are doing Satan's. Mr. Horner is a wily old fox and he knows that few people indeed are your classic cat-stroking villains, doing evil for the LOLs. If people believe they are doing something good or righteous, then they are willing to engage in some real atrocities.
Going forward, we will need to get seriously tough on those who radicalise vulnerable men (and it is nearly always men) with long prison sentences and putting them in special wings where they cannot turn others to their cause. We need an off-shore Alcatraz for these people.
We must also address real and valid grievances in the Islamic world - not because it causes terrorism, but because it is the right thing to do and it will eliminate some of the excuses. We must also recognise and make clear that most victims of terrorism, along with most of those doing the fighting are Muslims.
There is no talking with Daesh except to ask them what they would like for dinner as they serve their whole life terms - the death penalty is what these people want. The IRA never wanted the destruction of the United Kingdom, Daesh do.
I hope I don't have to write another post like this. I sadly fear I will.