This seems to be a bit of a gray area.
I googled “voluntary amputation”. Came up with links that didn’t really answer your question on the legalities, but there are certainly those out there who wish to do what you are suggesting.
There is a little bit of information about apotemnophilia, a type of body dysmorphic disorder in which the patient wishes to be an amputee.
This article describes a case of a UK doctor that performed 2 amputations, and (as you might expect) a flurry of discussion ensued.
It sounds like right now there’s a bit of an ethical dilemma. Hippocratic oath versus individual freedom, etc.
You’d probably be hard pressed to find a doctor to do it, and even if you did, you’d still probably have a bit of explaining to do. So it probably wouldn’t be something easy to do, but I guess it could be done.