Indeed £10k and even £15k is an absolute bargain for 150 hours of hour building in a multi crew twin jet aircraft. However despite the bargain that it appears to be when compared to hour building in a C152 or PA-28, ask yourself what the airline is gaining. Essentially, these airlines are doing away with the need of ever hiring First Officers. All they need to do is hire and pay a bunch of experienced training captains to sit in the left seat.
Hence, each and every one of us who take up such offers, we are actually quite directly putting in another nail into our own coffins. As by offering line "training" these very airlines who would ordinarily be looking to hire us will have no such openings in months and years to come. Why? they would much rather take on a new batch of cadets just churned out by flight schools all ready and eager to work for free for 3-6 months.
The regulators of European aviation are asleep and the unions pathetically scared to do anything about it. The choice is clear. Every single person contemplating line training without a firm job offer must reject and forget this idea.