Where is the PSTN swich off happening, and who is doing it?

Is the PSTN switch off happening across the UK, or just in some areas?

Everywhere in the UK eventually. Some towns and exchanges are being moved earlier than others, which is why a neighbour might be switched before you, or vice versa.

Who decided this, and who is doing it?

This is an industry-led change, with BT Openreach leading much of the technical migration as it owns and operates the UK’s largest copper access network. Ofcom is regulating the process by ensuring providers comply with consumer protection rules. The decision to close the PSTN was made by industry, not by Ofcom or the UK government.