The LED bulb can vary a lot in how they convert 230 Vac 50 Hz into a constant current supply, from simple capacitor to a pulse width modulated controller, the aim is to get as much light as one can without over driving the LED. If you do over drive an LED before it fails completely the output goes down, the current used is also dependent on cooling, so a manufacturing fault as simple as running out of heat sink compound could cause the lamp to fail.
The problem is when working out the life of a bulb they have say 100 bulbs under test and when only 50 remain working that is considered the average life, so unless you have a load of bulbs the same, it is hard to claim the batch was faulty, as often you only have one of the batch.
Clearly if it fails within days it's faulty. But if it lasts 5 years instead of 10 years you can't really show it was only lit for 4 hours a day and not 24 hours so you are unlikely to get a free replacement. So in real terms a £4 bulb is as good as a £12 bulb, as you are unlikely to get a replacement if either fails.
There are exceptions, I would pay more for a bulb for the caravan, running off a battery a bulb rated 10 ~ 36 volt DC with 100 lumen per watt is better than one rated 12 Vac 50 Hz with 70 lumen per watt, but in the house not worth the extra cost.