If you paid decent money for the car (over about £3k) then they should have at least 6 months life left (say 6k miles if you do lots more than that it's a bit different). This is the general period that you have with used cars in case of a failure that can be pushed back toward the dealer as a present fault at time of purchase.
Whether you paid decent money or not, you still have the same civil rights as a consumer. Its not 'a fault at time of purchase' either, the OP has been made aware the pads are thin – but still legal, and has accepted the vehicle.
IMO the garage/trader should of replaced the pads as good practice on their behalf (and maybe good will). But legally from the OP's explanation the trader has done nothing wrong.