"I also wonder how a Kenyan style attack would play-out in the UK."
Further to 500N's reply.
Several years when I worked for a southern UK police force we had a series of training sessions after the India shootings. As emergency call takers we were trained to recognise what then called an active shooter incident. We were also left in no doubt of that it would take some time for a proper response to be arranged for our county.
The nearest base for any of the armed forces is at least 70 mins drive and outside of our policing area. This does not including the time for callouts and getting the required kit etc.
The police would not have enough firearms officers to contain a shopping mall to any great effect and then it would take a while to get them to the scene as their would be in different parts of the county. You can't have all the eggs in one basket as their often cover any incidents. ie man holding wife hostage with a knife after a domestic.
In fact I would be bold enough to suggest that some countries that have a regular armed police with an army used to controlling their own subjects would be better prepared, at least in manpower, than the UK is.