So at the end, if I have been paying the full HP amount (£828 as above example), and after 60 months the car becomes the companies but been paid for in full by myself, thus a simple transfer of ownership to me would then render it mine?
Unfortunately, it doesn't work quite like that (though you may be able to come to an agreement close enough to make you both happy).
I think part of what's causing you confusion here is that you are focused on "salary sacrifice" as a thing that's bringing you advantages. In fact, it's just a label to describe something you could have done all along, and in fact tying the label onto it is disadvantageous - HMRC view it as tax avoidance and have
recently acted to stop people taking advantage of explicit salary sacrifice schemes, though fortunately ULEV company cars are exempt. The thing that's giving you the advantage here is that you are running a company car rather than a private car, and salary sacrifice is a way of organizing to do that in a way that's transparent - particularly where there are several employees notionally on the "same" pay grade and not all of them want cars.
Fundamentally, you are negotiating your pay/benefits package with your employer, and you now want them to provide you a company car. They have now offered you a salary of £X plus the use of a company Tesla; the fact that they might otherwise have offered you £Y without the car is not directly connected - maybe they are like
@ringi and don't like providing company cars unless there's a profit in it for them, maybe they are keen to have a green image and want to subsidise a carpark full of Teslas. The difference between £X and £Y has to be something in the region of the cost to the company of providing it - since otherwise either you or the company would consider it a bad deal and not accept it - but the exact number is a matter of negotiation.
If you have a company car, then it is by definition the company's car. They are responsible for insuring it (which might not be exactly the same cost as if you were insuring your own car), they are responsible for maintenance (which can't be precisely predicted - how many flat tyres are you going to get?), so there's no really precise right answer.
Because it is the company's car, the only way for you to end up owning it is if they sell it to you. And if they sell it to you below market value, then you are owing tax on the difference - just like any other thing of value that they might give you. If they decided to give you no salary but a free gold bar, that would obviously be tax avoidance, so it is with giving you a free (or reduced price) car.
Your problem is that you want to avoid any impact on the company's cash flow by paying (sacrificing) more than the true cost of providing the car, while the company is gradually gaining an asset on their balance sheet, then you are expecting them to give that back to you by giving you a free car at the end of it. So what you are actually proposing to do here is give the company a loan. Whether or not they really need or want this I have no idea.
In theory you could actually document it as such - you reduce your pre-tax salary by £X, then after tax take a further £Y that you loan to them, such that £X+£Y adds up to the HP payment; then at the end they pay you back the loan in cash and that gives you enough cash to buy the car off them for fair market value. But this is so complicated I doubt any sensible company would want to go for it.
Much more of a problem for them is likely to be the risk that you cease to be an employee or something else goes wrong before the end of the 5 years. They can (probably) sell the car and pay off the HP, but likely to lose money doing so in the early years.
I think much more realistic is to be sacrificing a bit more than the car is worth (but still a little less than HP+insurance+maintenance) so that the company is making a bit of a profit on it overall to compensate them for the minor cash-flow impact and the risk element. If you really want to own the car at the end, put by a bit extra in a savings account so you can afford to buy it from them.
Note also that there is provision in the company car rules for an employee capital contribution to the cost of the car, but this is all about reducing the BIK amount so is not helpful here.
Caveat: all the above is not professional advice, I hope it is accurate enough to guide your thinking but you should not trust it.