I can't talk about Scotland, Wales, or NI. But here's the data for England:
https://digital.nhs.uk/data-and-information/publications/sta...
There were 34,000 detentions under the act. Of these, only 4,150 were detentions following a Section 135 / 136 place of safety order (the bit of the Mental Health Act that the police can use).
The CQC also have their "Monitoring use of the Mental Health Act" report here: https://www.cqc.org.uk/publications/monitoring-mental-health...
This strengthens the point you're making. EG https://www.cqc.org.uk/publications/monitoring-mental-health... and https://www.cqc.org.uk/publications/monitoring-mental-health... both talking about the poor care recieved by patients as a result of the defunding of the NHS and social care systems.