I had problem with air berlin when they would not provide a lap belt. I very nearly asked to come off but we were underway and I tried to rationalise the situation. The crew said I just had to hold her tight.
When I got back I wrote to the CAA and was told they could/would not deal with the matter as they were not a british carrier and that the issue of lap belts was down to individual airlines. After that, never flew airberlin again and in any event bought a seat and took the car seat with me. Worked well.