The law says nearly the exact same thing in California (and I'd guess most of the US). Is there a substantial difference in the way it's applied in practice in the UK?

In England the most commonly used bits of the mental health act are Section 2 (28 day hold for assessment and treatment), Section 3 (6 month hold for treatment) and Section 136 ( a police officer can take someone from a public place to a place of safety for a 24 hour hold for assessment). There are also forensic sections of the act and there are community treatment orders.

Section 2 and 3 require two doctors and an approved mental health professional (AMHP). One of the doctors has to be specially trained in using the MHA. The other doctor should be someone who knows the patient. The AMHP is usually a social worker who knows the mental health act law and is there to protect the rights of the patient.

There are legal requirements placed on the patient's local MH Trust. One of these is that when that patient's nearest relative tell the trust about concerns and asks for an assessment under the act then the trust must consider that request.

A person detained under the act is provided with an advocate from an independent advocacy service. This is free legal advice. If they feel they are being unjustly detained they can go to tribunal to get the section overturned. And that process is also open to relatives too.

There are no fears that your loved one will be shot and killed if you involve the police.

I don't know anything about the US law, but it seems somewhat complex and expensive. There are laws at the state level, and at the county level. Some states say that the harm to self or others must be "imminent" which is a pretty high barrier to meet. Most states allow a forced treatment in the community option, but California requires the counties to adopt that law. So for California where you live decides whether your family can ask the courts to force you to take medication in the community.