So it depends on the section (there are loads!). Under Section 135 police have powers to enter the home and take the person to be assessed by an approved healthcare professional or doctor (so yes, in the UK in a similar situation, there would have been a lawful intervention). Under section 5(4), nurses can detain someone in hospital for a limited time, pending assessment, under 5(2), doctors can do the same. These are in emergency situations. In non-emergency situations, family members, the GP etc can raise concerns and request an assessment.
Under formal assessment by a specialty trained doctor, someone can be sectioned for 28 days under Section 2, and up to 6 months under Section 3 (and this can be renewed).