How many people get killed by the police and in UK state custody?
The rule of thumb is 1 victim per week killed by the police forces of England and Wales alone and at least 10 more in UK state custody dying an untimely death (i.e. inside prisons, mental health institutions, detention camps etc.).
The NGO Inquest offers detailled statistics of police and prison deaths derived from official numbers for more than the last decade.  Official 'I'PCC figures add up to almost 600 deaths in custody during the last 3 years.  According to a recent study called Annual Report 2006/2007 of the Forum for Preventing Deaths in Custody consisting mostly of official departments and commissions, most of these deaths could have been prevented . However, since the victims are all poor and / or black
Who gets killed? And who doesn't?
The first recorded death in custody case was a homeless Nigerian immigrant named David Oluwale who was found dead in a river in Leeds 1969. His case was also the one and only exception where any officers were convicted, tough only for assault charges and NOT for manslaughter, let alone murder.  Only recently a 2nd book was published on this case, benefiting from newly declassified documents.  As the NGO Inquest points out, 'a disproportionate number of Black people and those from minority ethnic groups have died' in state custody.  However, as INJUSTICE co-director Ken Fero  explains, in absolute numbers the majority of deaths in police custody in the UK remain whites, though black bereaved are more likely to start campaigns demanding justice. What all victims of death in state custody do have in common, is that they belong definitely to the class of the 'havenots' and mostly live in tough inner city areas. Cause what would happen if the police started killing one rich white resident of a posh area after the other, guess you can imagine yourself ... Inquest speaks of 'institutional racism in the criminal justice system' 
And by the way, did you know that 'within mental health settings', where again black males are disproportionately affected, torture a.k.a. 'Pain Compliance' is being officially considered as O.K., too? 
To date there have been 1743 deaths in police custody or otherwise following contact with the police in England & Wales since 1990
For a breakdown of deaths in police custody or following other forms of contact with the police, as well as vehicular pursuits and road traffic incidents (RTIs) since 1990, by calendar year (January-December) and type you can review here : https://www.inquest.org.uk/deaths-in-police-custody
You can also find further breakdowns of this data on the following pages: