Don't put £12.5k down as a deposit, you would be mental too.
If you need advice off anyone, I can probably help you most because I have been in your position for a long time trying to chase cars and I've had about for failed finance applications in the last attempt (RS3).
If you were going HP, yes.. but PCP, no!! If you then in 3 years time decide you don't want to pay the remaining amount, which will be around £20k.. you will lose your £12.5k, plus your monthly payments! Yes your payments will be lower, but that's a crazy amount to put down.
The maximum you ever want to put on a PCP is £4k, because that money you never see back.
Put down a £3000-£4000 deposit, use the rest of your pot to afford the monthly payments.
Are you in a financial situation where you can 100% afford it or are you on a fine line? I were closer to the fine line when I wanted an RS3. Right now I spent around 2/3 of my wages a month, 90% of which on the car. If you are needing to calculate fuel, base it on the car doing 23mpg and not the maximum 30 that people quote. That caught me out, I allowed a maximum of £250 a month fuel expecting to spend £100 less than that, I've used it all.
PM me if you like, I don't want to disclose exact earnings etc on a public feed, but I will help you out as much as I can to make sure you can afford it and don't put too much money down.
I do love this PCP myth....
You are both financing the same amount of cash, whether you pay a large deposit and small monthlies or small deposit and large monthlies is largely irrelevant, you will both spend basically the same amount of the finance agreement! You both lose!
I say basically, as the person with a higher deposit actually pays a bit less as he has slightly less interest, so if anything a larger deposit is better. When you consider a savings account will earn you 0.5% at the moment, putting down £5k and keeping £10k makes zero sense unless you have somewhere else to invest it. Might as well save £10k worth of 6.3% interest and increase your deposit....