Ofcom published the document Spectrum available for Powerline Carrier Systems

The Energy Networks Association may make assignments for power line carrier systems in the bands below. These systems fall into two categories –

• Normally Quiescent Carrier Systems. These systems may use any frequency within the range 30·0 to 526·5 kHz.
• Continuous Carrier Systems. These systems may also use frequencies within the range 30·0 to 526·5 kHz.

Download - https://www.ofcom.org.uk/__data/assets/pdf_file/0008/103301/fat-power-line-frequencies.pdf

Ofcom Spectrum page - https://www.ofcom.org.uk/research-and-data/technology/radio-spectrum