This happened to me a few times but I never thought asking for avios compensation. How do you go about this, do you ask the cabin crew on the plane or do you contact customer services afterwards?
I had a few small issues on a recent flight in CW, amongst one of them a dodgy touchscreen on my IFE. The flight attendant was very dismissive so I didn't feel comfortable asking him for any compensation, but filed a complaint online (combined with the other issues I had on the flight), and got 30K Avios as service recovery. Which, incidentally, was 5K more than it cost me to upgrade from WTP in the first place!
Like other posters have said, it may be more efficient to ask the flight crew directly if you feel comfortable doing so, but online certainly is a perfectly serviceable alternative if you'd prefer.