One thing that people should bear in mind: The data used in that article is hilariously wrong.
The overall idea is fine and could certainly be supported with proper research, but the article does not do this. Here is a comment I made about this earlier today when this same article appeared on /r/conspiracy.
Well, 5 times the population is a bit irrelevant considering the numbers of deaths and timeframe presented by the article
HOWEVER
The article seems to be basing its information off this wiki page http://en.wikipedia.org/wiki/List_of_killings_by_law_enforcement_officers_in_the_United_Kingdom
I find it a little unreal to believe there were no deaths by police between 1920 and 1977...
Here is some better info: https://www.ipcc.gov.uk/page/deaths-during-or-following-police-contact https://www.ipcc.gov.uk/page/deaths-during-or-following-police-contact
Here are 2 articles that give some other insight -- and note that the deaths are NOT necessarily the fault of the police, AND the fact that the metric is one that is tracked and publicly scrutinized):
1433 related deaths between 1990 and 2011 = 65/year (though the rate is lower in the later half per the next article)
333 deaths between 1998-2009 = 30/year
So it seems that the number per year is even decreasing. And again, these are including deaths that aren't even directly the fault of the police.