I would suggest that some limit on student PPL flights is sensible. Not necessarily from a fuel planning point of view but from a student fatigue point of view. It is often noticeable that the performance level drops the longer the flight.
Personally, I've lasted about 3 hours in a 152 before becoming very uncomfortable.
On the leaning side of things, I agree that it's a skill not taught well in the UK. But an important note made earlier is that you should consult your POH for your airframe/engine combination as to how to do it properly. There are instances where incorrect leaning have meant fuel burn is vastly different to what was thought.