There are certainly private “lifestyle” doctors in London who do this - maybe not ketamine, but definitely sleeping tablets and opioids.
The two doctors I know of who do this both failed out of mainstream medical training at SHO level, ie incredibly junior and shouldn’t be working in an unsupervised setting. One set up an online practice, the other is the in-house doctor for some sort of wellness clinic (massage, injectables, and then her prescribing drugs of abuse). I have literally no idea how they revalidate, but I guess their RO is probably also dodgy.