Drugs lab/den no. Simply put you'll either end up with some rival dealers/producers getting their information wrong looking for people, chemical poisoning or police raids at 3am with a semi-auto being shoved in your face.
Murder victims house? Probably not an issue, probably lived in places previously owned by some form of violent criminal or had violent crimes up to and including murder inside of. London's quite the old city. You're not likely to find affordable new builds here. They all tend to be "luxury apartments" which cost way too much for what they are, £200k for a 1 bedroom studio apartment? As my midland born ancestors would declare "like fook!".
Natural causes deaths wouldn't in the slightest bother me.