Aye, excess mileage is only applicable when handing it back, so not buying it with the final balloon payment.
It can also effect the ‘trade in’ value if you choose to PX the car against another/new one but just review what older cars go for of your make and model with the same mileage to work out what the PX value you might be on your expected actual mileage rather than the mileage you choose on the PCP.
I chose the lowest annual mileage because I intend to trade in for another at the end of it and I know that cars I have with 100k miles at 3 years old are going for what the final payment would have been anyway so the trade in value will be same/better as the final payment. By doing this I kept the monthly payments down and save a few quid in the end. I expect to put on around 36k miles when trading it, which is double the actual contracted mileage.