Whats to stop the OP saying he is doing 10K a year, which will bring the monthly payments down, put some of that difference away each month into an account. When he comes to change the car the xs mileage will only come into effect if he is handing the car back and walking away.
If he is trading it in or selling it privately the money he has put by would maybe cover the offset of having a higher mileage car than average and hence less of a deal.
Or is it just swings and roundabouts?
If you quote low miles to get a cheaper price per month, you have to be putting money away for when you trade in, or you'll be in negative equity. No way a GTD or GTI in p/x in 3 years time is going to be worth £2700 more than accurate GFV, you will have to pay the piper at some point. If you're putting money by to make up the shortfall later then what's the point? You are paying more interest on that would-be payment deferred than you would accumulate in a savings account, you might as well pay it up front.
OPs spec is £26224 on DTD.
With deposit down, he'll be financing £24724, with a GFV of £11770 (25k miles PA declared) and paying £463pm. That car might end up having a p/x value of £1000 more than GFV. If you want this car then you're going to have to chase a broker deal to make i work in the budget you have planned for.