Definitely wouldn't recommend overpaying to "build equity" - put it in your savings instead.
My wife has a Mini Cooper Clubman which is currently £5,000 in negative equity a year before the contract ends - I'm not bothered, we'll hand it back and walk away.
PCP is great because the risk is on the finance company rather than you - even if the car is worth less than the GMFV (balloon / guaranteed minimum future value) at the end, as long as the vehicle is in a condition commensurate with the age and mileage it's not your problem
...this is also the reason why there are some used car bargains out there for a "cash buyer" as the manufacturers are propping up the new car market.
@TGR companies will do up to 30k a year, just pick what will suit you as best you can, but don't stress too much - the penalty for going over is usually about 7p a mile so even if you went 10k over on mileage you're only looking at a penalty of £700