Useful next steps
Avoid continuing to use a fixture that is overflowing. Keep people away from wastewater and tell anyone assessing the problem about any drain-cleaning products already used.
In a shared building, tell the managing agent if multiple flats or communal drains are affected. Do not assume the blockage is confined to your own pipework.
Check with your wastewater provider if a public sewer may be involved. Responsibility and the right reporting route depend on the actual pipework.
To identify the appropriate water or wastewater provider, use Water UK's postcode lookup. A town name alone is not a reliable way to choose a provider.