Candidly - Dont buy a Model S new or used now.
It pains me to says it, but that is the result of Tesla's price slash.
There is now horrible depreciation built into Tesla ownership.
You dont really think a 2yo ModelS P100D is seriously worth £85K when a new one is better and now costs £95K?
ModelS used prices will fall steadily until the market realigns, and there's a load more about to come to market with Model3 starting too.
Even new cars now will suffer as the faceilft is puportedly September '19, then new batteries next year, then a whole new model a year later.
Sure eveyone always has new stuff, but Tesla is techno gagetry so expect similar depreciation to your phone. Tesla have also proved themselves unreliable as to new prices so this has to be factored in too.
This is precisely why the financial markets are hammering GFVs, it is entirely of Tesla's making and is a total own goal as low cost of ownership was one of the biggest selling points.
As
@Wannabeanowner says new a Model3 cash purchase is probably the best place to be just now as resale should be sound at least early on.