I go to a dentist in Harley Street, but not because he's in Harley Street.
We're still on US insurance, so when my husband had a dental emergency soon after moving last year, he just looked through our plan's directory and picked a name. The dentist happened to have his practice in Harley Street, and it's very close to where we live. He was happy with the dentist, so I went there, too.
I'm happy to give you his name, but I have very uncomplicated teeth (no fillings, braces, etc.), so I can't vouch for anything other than some x-rays and an exam and a hygienist cleaning. It was pretty much the same as any good-but-routine appointment I had in the US, but I have nothing else to compare it to in the UK.