Police officer here (not in the met).
I am beyond horrified at what happened to Sarah Everard and incredibly shamed that the man who murdered her was a serving officer paid to protect people like Sarah. I am angry for what he's done to Sarah and the damage he's heaped on public confidence in the police, particularly among women.
I would love police to be double crewed. It would lead to a lot better decision making IMO (chances of 2 officers making a bad decision much reduced). It would also reduce the number of steadily rising assaults on police officers. Not going to happen though. The area I police is staffed with 4x fewer officers than 20 years ago and we're failing to meet demand as it is. There's no money or political/public will to uplift numbers.
I also don't think it would help. My force, and most others, already have a policy that 2 officers have to transport to custody. 1 officer may attend and arrest but another will then be needed before anyone can leave the scene. This is because the driver can't monitor the prisoner (there have been cases where prisoners have died en route). Most members of the public don't know this even though it's been in place years.
Most people (and especially those with typical female socialisation and those who don't have previous interaction with the police to draw on) would just see the warrant card and capitulate. In a country where we police with consent, that's actually how it should be - with the caveat that the police absolutely must be acting with integrity and relied on to keep people safe. That wasn't the case here which is why it's so, so awful.
There will always be rare exceptions of incredibly clever sociopaths, but what's actually needed (and shouldn't be beyond the wit of forces to achieve) is a robust vetting process, regular, mandatory psychological assessments for officers (which doesn't currently happen but IMO should, which would not only pick up red flags but might also help with their decreasing mental health levels) and some accountability. We already have a Code of Ethics making those who had concerns about Couzens duty bound to report him. Whether they did, and what was done about it, remains to be seen but that's where the real questions about preventability lie in my view.
In the meantime an innocent young woman lost her life, for which I am truly sorry.