The PCP is only a finance contract, you can break it at any time, be that in month 1, month 10 or month 30, all you do if you want a new car is P/X the car in and clear that finance agreement.
The longer you are into a contract, the more likely it is you will have some equity in the car and vice versa, the shorter you are in contract, the more likely it is you'll be in negative equity. All depends on the deposit you initially put in and the value of your car at the time.
For example, with the guy above in the S1 and his £500 deposit, he may well have had equity in his past PX car + his £500 putting down a high initial deposit or he may not have any equity in his S1 going onto his new car but also small or no discount on the new car.
Dealers are sneaky and never lose out.
For example, let's just say...
Audi S1 - Finance owing £23k
Cap/Trade/P/X Value is £21k
Dealer clears the £23k outstanding balance on customers finance agreement to clear the slate
Dealer has already contributed £2k to the deal on the new car
Audi S3 - £36k (Normally you would get £3k off this as discount) but due to the £2k above, you only get £1k discount, customer pays £1k deposit himself and finances £34k on his £36k list price S3.
Customer comes to trade the S3 in at 12 months time but finds the CAP value is only £26k and outstanding finance £31k, there's now £5k negative equity, customer has to now pay a much bigger deposit to buy himself out, dealer may help out again to the tune of £2-3k (But that's taken from the discount price on the next car) which means you end up paying full list price on the next car, you then get yourself into even more negative equity 12 months further down the line if you decide to trade in early again
That is the way short term PCP's normally work out, but not always, it really depends on the car. The S1 is rare and values hold up much better than S3's do. I suspect if the poster above tries that again after 12 months with his S3, there will be mega amounts of negative equity.
All you need to be aware of is the outstanding balance on your finance agreement and the value of your car. You can do this by going to we buy any car, putting in your registration and you'll find out your trade value of your car. This is the figure your Audi dealership will use, give or take £500.
Had I kept my car within the mileage terms of 15k the negative would be subsequently different. 
