You'd owe them a ton of money for excess mileage...
That's the point Ste, i don't think you do, under voluntary termination rules you only need to take reasonable care of the car. The sources I've read make no mention of the excess mileage clause. Only that you can terminate if you pay 50% of the total balance owing.
I agree, I dare say Al will have the answer
PCPs, being a form of leasing, have crippling mileage charges... On a £70K car, I would expect these to be around 35ppm
Best bet would be £1,000 deposit, or more, but your only throwing any extra away when you return it, and 48 months straight HP. Once you have made the 24th payment, ring the finance company and ask them to collect the car as you wish to voluntarily terminate the agreement as per your right. They can only charge you a penalty if the car has damage/not been serviced/is full of river water/not roadworthy. Expect to spend a few quid removing any sign of the mileage- broken/damaged trim/wheel refurb/proper Valet. Stone chips proportionate to mileage are fine, busted undertrays and barely legal tyres are not.
If you put down £40k and HP the rest over 48 months, you would end up paying £55K plus interest at the 24 month point, whereas with £1k deposit, would see around £1,500 per month, leaving a halfway total of around £38k. Do the maths... Higher monthlies vs greater deposit and place the order accordingly.
Incidentally, you can return the car at any point before halfway but you are liable for halfway less sum paid... Equally you can return without prejudice at any point after halfway, although doing this will cost you the residual value. Fine if you're talking mainstream hatchback, but Tesla residuals are strong (hence the favourable lease/PCP rates).
Remember also that the residual value only becomes real money when you sell it, assuming of course that the finance is clear.