Here is information on making complaints to IPCC
They have guidance just read the section and you will see link.
Here is link to MPS complaints
You can make individual complaints and also organisational complaints.
Those directly affected by the restriction of medication , food and drink should individually complain.
It will take time for the complaints to be processed so I suggest people do this asap.
If you need help making a complaint they can approach MP, Citizens Advice and some DPOs may assist too.
The MPS has already launched an inquiry into policing at the Westminster demonstration