Rudder trim. Unfortunatley the Warrior is so easy to fly without virtually any rudder input that it really instills bad habits in students. I recently converted to another type and was surprised at just how important it can be to keep the ball in the centre. I can understand you not using the rudder trim much (many types don't have it), but I'm not sure it was a good idea to activley tell you to ignore it! Please, at least check it's centered pre-flight, if nothing else. If someone's been flying crosswind before you, you could get a nasty surprise on takeoff.
Mixture. I too was surprised not to be taught this during training. I know we don't get chance to fly higher thasn 3000ft much in the UK, but a PPL is a license to fly just about anywhere in the world and this should be an integral part of the course, surely? I recently flew in the USA and had to ask the checkout instructor to demonstarte mixture leaning as we were regularly getting up to 6000 or 7000 ft. Without fail, I forgot to reset the mixture during descent - because I'd never done it before and my UK checklist doesn't have a 'descent checks' section.