What is the difference between 'Homicides per million population' and 'Homicide Victimisation Rates (per million population)' Because the figures for homicide victimisation rates are lower here http://www.scotland.gov.uk/Publications/2000/03/2ff917ae-270d-47d9-acd0-f89c30688868
That is detected - Homicide figures since 1900 I suspect we are better at spotting homocides now and many previously were unrecorded
So are the figures on the scotland page, it has england and wales in one column and scotland in the other to compare the two (scotland rates are much higher apparently) but the figures for england and wales are different to on the link you gave. It's also worth noting that 'homicide' includes manslaughter and while i've no idea of the figures there seems to be more people convicted of man slaughter now for workplace related accidents, negligence etc than in the past which could account for the increase.
I should have pointed out on the original link that it had English figures on it but I kinda forgot. oops. It would be interesting to see a proper breakdown of the figures, showing how many are actual murders, how many are manslaughter due to violent crime and how many are manslaughter due to negligence or something else where there is no intent to injure somebody.
Another possibility. The marked increase came about in the mid-70's, when we started getting more violent films and TV programmes. I don't think you can attribute it to any one thing.