Lots of people have made good points, but here's a few facts. The cost of training has increased significantly in the past few years. L3 are a leading provider worldwide, and their current ATPL training course is £90,000 - not including any accomodation or food etc. With a type rating the cost is closer to £125,000 (www.l3commercialaviation.com/airline-academy/easa-pilot-training/easa-pilot-training-courses/integrated-atpl/)
Most take a loan for this amount, and the going rate is just above 4% interest.
Thus, over the course of a ten year loan, paying back £1000 a month, the total cost of training is something like £114,000, not including any accommodation or food. If they pay for their own type rating, the total cost with interest is approx £150,000.
The lower amount alone is enough to go to university and pay maximum fees for over 12 years. It comes with no guarantees. If you can't get a job, or lose your medical license, you have to pay it back all the same. Over a ten year term you're paying £1000 per month on your training loan regardless of whether you have a job or not.
You also suggest that they are asking for a £20,000 pay rise, which means you think that the average pilot earns £200,000 per year. This is absolute insanity. Such pay just doesn't exist in this country. Pay scales between most airlines in the UK are broadly similar (BA pilots actually earn less for most of their career compared to Easyjet/Ryanair/Jet 2).
Also note that the starting basic salary for a cadet pilot at most UK airlines is in the region of £25,000.
Imagine owing £150,000 in training debt, earning £25,000 before tax and having to pay £1200 a month just on your loan repayment.
So when you say pilots are asking for another £20,000 a year, think again.