As a mere former PPL student, my experience is that defects are a way of life, and one is in a poor position to query them, short of walking out and going to another school. There is a lot of politics involved in that, because schools tend to stick together and don't like students doing this.
The previous school might "lose" some previously passed exam results, to make a point
The CAA 50hr checks mean nothing. I have flown planes with bare-end wires hanging in in the engine compartment, and the FI didn't give a damn. The checks just make sure the controls work and the wings are screwed on, more or less. The service firms are under pressure from schools to do a quick job and do it cheap, so only the absolute minimum gets done.
The industry gets away with it because structural/control failures are extremely rare in fixed-wing planes and as a result the planes are just about impossible to get to crash by themselves - the pilot has to do it himself
Even the most battered 1970 C150 will be safe to fly, and it would probably be safe even if it didn't the inside of a workshop for many years; with just a squirt of WD40 here and there. And if you started with a new plane, it's unlikely that it would develop a dangerous fault for 10+ years.
Same with cars; structural failures barely feature in accident stats. The real difference between aviation and the motor trade is that the former cowboys are tighly regulated.