Domestic homicides – that is murders and manslaughters where the killer and the victim are either partners or family members – tend to account for 20 per cent of the total every year.
However there is a lot of variation over time which does not necessarily follow the general trend. For example, in 2017 there was a peak in homicides at the same time as a lowpoint in the number of domestic homicides (6.6 per cent of the total).
The graph also makes clear that the number of murder and manslaughter cases does not wildly diverge each year. It should also be noted that domestic homicides tend to be murders (there were only two domestic manslaughter cases over this seven year period).
These statistics all come from a readily-available disclosure by the Metropolitan Police in March 2019 (linked here, though they are often deleted after an unspecified period of time).
Though this disclosure does not distinguish between homicides committed by partners and other family members, in our experience most domestic cases involve intimate partners or ex-partners.