@People's Front of Judea I suspect the reason they don't do CH, is because they can't fund it. Merc/BM/VW et al are sat on piles of cash, and healthy positive cash flow. They also have captive finance companies that are there to help them shift volume.
Tesla are burning cash at the moment, and the big plus with a third party lease is the cash is instantly recognised in Tesla's accounts.
Now they could use a third party CH company (or you could make your own arrangements) but list price to list price you won't get nearly as much car for the monthly payments. The finance company is taking all the risk, and the RVG isn't open to them. To make matters worse they need to make their margin on top, and don't get discounts on new vehicles either. (Just look at an A7 3.0 which is similar list, it's half the price on BCH)
Having bought for cash, with a lot of press around the 50%, and seeing what the US market looked like I was pretty confident in seeing at least 35-40%. However as the constant flood of upgrades have happened, and a brand new car now is effectively 20% cheaper, I can't see this happening.
Now no doubt Tesla will try and buoy the market by buying up all the used cars, a trick Porsche have been doing for years. So it's likely that's where mine will end up, but I'm not holding out any hope it will get near it's 50% after 3 years even with < 25k miles on it.
TBH it has really put me off to the point where I don't think I want another Tesla in a couple of years time, and even if I did it wouldn't be new. Once bitten and all that.
Saying that maybe all these delays we've been having on UK D cars are the start of Tesla showing a bit more sense as they have become more sensitive to the impact of cavalier upgrade strategies. I'm sure Tesla UK didn't want a repeat of the autopilot hardware debacle again!