There are three stop lights on the back of a Disco. One on each side of the vehicle and a high level brake light at the top of the window in the rear tailgate door. If that one is working then it's a fair bet that the switch and wiring are OK so you should start your faultfinding by replacing the bulbs. Make sure that the connection in the bulbholder to the body of the bulb is clean and functional.
Regarding your headlight aim, the purpose of the MoT test is to examine the vehicle in the condition in which it is presented for the test. Being pedantic, if that was all, then it could have resulted in a fail followed by a repair bill and then another retest, but since there were other "reasons to refuse" a test, he just handed it back with the bill for the test fee for you to repair.
You are quite within your rights to discuss with the tester the reasons why he failed what.
He might have thought that the hazard lights switch should have lit up with the flashers, but on the Disco the only indication is the two direction flashers in the instrument binnacle, but you might have to provide some documentary evidence say from you owner's handbook to substantiate your claim.