What I don't get in this whole discussion is why people are really so afraid to write up problems with the aircraft. It almost seems as if a defect that's been put on paper is an immediate cause for indefinite grounding of the aircraft, without exception. And that's definitely not the case, as far as I'm concerned.
There is a culture in the UK PPL training business of doing the absolute minimum maintenance. And in UK GA there is a lot of airfield politics, with a lot of backstabbing going on.
A lot of defects are not enough to ground the plane; for example when I started training I complained that the fuel gauges were completely useless. It was carefully explained to me that all spamcan fuel gauges are completely useless. As an engineer, I was a bit shocked... but the school was right in stopping me making a report. Now I know this is true... all the old fuel gauges are indeed almost completely useless.
A lot of avionics are INOP and this is completely normal in the training scene; it is OK for any amount of nav gear to be duff, in a plane which is used for PPL training. The legality gets a bit marginal if the said plane enters IMC, but in theory this never happens (the little bit of IMC experience in the PPL can be done under the hood). But if it did happen (which is very possible if the plane is rented out, etc) and there is evidence of illegal equipment carriage... luckily there is no known case of the UK CAA ever going after a school for anything like this (or anything else?). If nothing is written down, everybody is safe.
A lot of planes have defects which really should be fixed immediately but it is more convenient to defer the fix to the next service, when the MO is quietly asked how much it would cost, and is there a cheaper option, etc.
I used to get really p1ssed off with what went on, and I was glad to get that piece of paper, so I could rent something slightly better, and I bought my own plane as soon as I got a chance. Now I see things are a bit better in that there are more options, with some schools operating better quality hardware. I think 10 years ago was a low point in the UK PPL training scene.
As a renter you can report stuff, and if you have a choice of places to rent from then you have more options. But almost nobody will do the proverbial on their own doorstep.
As a based owner you cannot take any kind of risk, especially if hangarage is involved.