Wanted to respond to the original question of whether to buy from Tesla or an independent dealer...
Personally I think that the most important factor is whether you can find the car you want with the spec you want at a price you are willing to pay. The best way to gauge what's available is to use this website:
Tesla MS and MX cars for sale in the UK
That site is fantastic and will show you what's available, price changes and the cars to avoid based on ridiculous pricing.
I originally purchased a CPO car from Tesla but was so disappointed by its preparation on collection that I declined to take delivery of the car. At that point I nearly stopped considering buying a Tesla at all...
However soon after, I located an alternate Model S through an independent dealer which tuned out to be a newer and much better spec'd car for little more than that CPO car I had intended to buy. I am very happy with my Model S! In fact I absolutely love it and will never buy an ICE car again.
Things to note are that the free supercharging and warranty are transferrable so you shouldn't have any concerns over this point. Also at 50,000 miles it is possible to purchase the same extended warranty for around £1500 (I believe) that you get with the CPO cars. So if the car you intend to purchase is below 50k miles and 4 years old, you always have that option for the future.
If you can find a CPO car at the spec and price you are OK with then buy from Tesla. If you find an alternate from someone who specialises in Tesla's then I would be OK purchasing from them too. Personally I would also be OK buying from a private seller too provided you take the usual precautions when buying any car from anybody.
My advice would be to avoid pre 2015 cars, cars without autopilot, and to ideally choose a dual motor car over the single motor ones and the largest battery you can afford. Avoid high mileage cars because of possible warranty issues. The other options are really up to you. Always buy the best spec'd car that you can afford as it will always be easier to sell in the future.