Contacted the PDSA about how to go about putting Bellatrix to sleep. Tried calling but since I don't think it's an emergency, they are closed to non-emergency calls.
Going to try and make her last few days as enjoyable and luxurious as we can.
https://www.pdsa.org.uk/pet-help-and-advice/looking-after-your-pet/all-pets/when-its-time-to-say-goodbye