IO540
If you really encountered such bad instructors, then you should have had a serious word with the school. The UK has a reputation for the best quality of flight training around. I work for a flying club and an FTO which teaches instructors. I haven't encountered a really poor instructor yet, even those with whom I disagreed about the training scheme. I have had to check out several for the club I fly for. Where did you find such poor instructors?
I must say some of the rest can happen, though an unbooked lesson would never push out a booked one anywhere I have flown, and timekeeping is only bad when things go awry. Then if you are only willing to pay your instructors £10-£20 per hour for his or her time, someone who has expended a minimum of £50,000 to qualify, then your instructors will book to fly every hour they can. If they are then delayed there is no flexibility for your convenience. If you want better service there, then pay them a better rate. While I try to adjust my life to fly with every student when they want to fly, I certainly bend over backwards for my students who give me the extras, from gifts to simple respect and enjoyable company.
Neither organisation owns a small, 1960s "coke can", both offer tuition on very capable aircraft built in the 80s or 90s (although one offers a 1970s aircraft at a very competitive price - it is a pleasure to fly, though not so muh as the 1952 Chipunk I flew a while back).
If you've never had GPS failure, learn about GPS. It is not infallable, I know people who have ended up with blank screens. Even the US military admit it is not what most pilots imagine it to be (I do some consulting for a GPS manufacturer, so keep somewhat up to date on the literature in the specialist press).
The slide rule is a good piece of kit, better than an electronic equivalent in at least two respects if you bother to learn to use it properly. Radio navigation and GPS is a good backup. However you cannot rely on it, it can give false or poor readings, pilots who have not had full courses on instruments persistently use it incorrectly. When I give cross-channel checks I insist that one way the student uses no internal radio navigation aids (VDF and radar fixes acceptable) to show that they can cope with radio nav failure even outside of sight of land, just by following the plan, and to show that the plan works, so they don't blindly believe the wrong radio information. If I have a GPS (do on one plane I fly) I don't use map mode, as this induces terrible navigation techniques, as I have seen in PPL holders I have flown with.
I don't scrap any where near 50% of planned flights do to weathre, despite never flying on instruments (too out of practice to be safe) and flying around 30 hours a month. Over a year maybe 20% or so.
To actually answer the thread:
My students have maybe taken 3 or 4 bookings to get a QXC. The weather really does need to be ideal. However the same applies to reputable schools in most of the world. There is nowhere I know that has ideal weather - I did my PPL in North Carolina and cancelled a lot of days due to rain, and flew in Florida nicking bookings off grounded PPL students supposed to be solo.