PCP's are an insurance policy as well. You get a value that's guaranteed at the end of the term, so although you pay interest on the 'balloon', you are not liable for it. Even if the car is worth less than the GMFV, you simply hand it back and walk away. The aim is to get your deposit back at the end of the term to go into the next one. So as has been pointed out, the key is to get the monthly to equal the expected depreciation. You can however put in a low deposit and 'pay-as-you-go' on the depreciation and maybe lose some/all of your deposit, or put in a large deposit and put in the depreciation up front if a lower monthly is your priority.
Now some manufactuers have had very tempting PCP deals, low customer deposits, high manufactuer 'contributions' and low monthly payments with high GMFV's. But with these you seem to go straight into negative equity as soon as you drive out the dealership and for many peeps they are trapped in the car, being unable to buy their way out and having to stay to the end of the term. LR is different, the GMFV's are lower but the vehicles have strong residuals because of demand for them, the deposit/monthly is up to you, but you tend to retain what you put in at the start. Recently the industry has had to lower their GMFV's across the board, as it was noted that too many people were having nothing left at the end of their deals. I.e. Payments too low.
So with the DS in particular why do a PCP over buying outright? Tbh, on this car buying outright is a sound decesion as I don't believe there is a risk of their values tanking overnight (unless either of those Ball's blokes get in & decide that as well as a mansion tax, there needs to be a 4x4 tax!) But with a GMFV of around £20k on these cars, you could instead see if you can do something better with that money over the term investing in the markets to offset the interest & hopefully make much more! Buy a boat? Up to you!
I think c.£500/month on this car will be a good 'average', but probably not for the first year, when I expect values to be remain strong on this new model.
Only at the end of your ownership will you know how well you did. (NB You can sell the car at anytime as it is yours. Personally never give the car back to finance company at the end, always sell private or part-ex, you will get a much better value than the finance company who will interpret 'fair wear & tear'differently to you!)