Whoa! Don't get your wallet out just yet mate (spoken like a true Yorkshireman!). I am assuming that the "warped disc" showed up during the brake test. As far as I know the only certain way to ascertain whether a disc is warped is to put a dial gauge on the fork leg, rotate the wheel and measure the run out which can then be checked against the manufacturers allowed tolerance. This does not form part of the MOT.
If, as I suspect, the problem manifested itself on the brake test when the tester felt a pulsing through the brake lever then the cause could be glazed pads. I recently was getting a pulsing through the lever when braking and even a slight judder at the front end when coming to a halt, which was getting progressively worse. The cure (as suggested to me by Keefyb) was to have the pads out and clean them up, either to cure the problem or at least eliminate them from the diagnosis. Out they came, pad faces rubbed with some coarse emery, finished with a steel brush, and the backing plates cleaned and a dab of copper grease smeared over. Took it for a test and hey presto! – no pulsing and no judder.
If however, you do end up needing new discs I was keeping an eye on a set on ebay (just in case). The seller is siren110876 and the auction finishes on 8th May.
