The answers above have probably summarized it well enough, but my try goes like this:
"I assume that £80k is a reasonable estimated of earnings for the guys with years of experience". No, it´s not. After 16 years in the air, I´m not making that; and won´t be by the 17th year either. You may mean: years working for the same airline, but to get to those "years working for the same airline" you´ve had to go trough many more years of instructing, banner towing, chartering miners in Congo on a Year 250 b. C. Cessna 206, some more banner towing, one year working at a bar, back to Congo, farming in Sunny Spain for some months... making under £20K/year. Then, if you´re one of the lucky few (that are also looking for an airline job, since many of us are happy flying something else), you may one day earn your £80K. It won´t be after years of experience, but after many years of experience, suffering, continuous studying and a truckload of luck.
Back to the "studying" part, it´s not just the two years to get the licence. You have to study and get your "licence" for the plane you´re gonna be flying as soon as it´s a tad bigger than a Seneca. Study to get it and then stay current. "Should I pay for it myself besides having paid for the whole CPL/ATPL?". Well, when you´re making £20K/year, it´s an easy decision, with one condition: you have to be able to survive two years without food and living under a bridge. Not many companies will hire you without that other "licence" for whatever planes they use, so unless you can survive those two years, you´re not gonna be in that "those that make £80K" group.
Then again, is that being over-paid? Remember that they´re not being paid that because of what they´ve suffered/studied/su***d to be there, but because of the income they generate for the company paying those wages. If my morning disposals at 'le toilette' were worth half a million quid, I´d be paid 80K for them, whatever my effort to get 'the job' done. The companies don´t pay based on your efforts or the praiseworthy of your job, but based on the income you generate, period. In this case, besides, the pilots getting the now-famous £80K happen to be people who studied, paid an awful lot of money, spent years surviving by eating cookies in a caravan, can´t have a normal life, spends days away from home (looks cool, but it´s not THAT cool once you´ve made it more than twice), can´t have normal relationships if not with other pilots... Their whole life is dedicated to the company.
Is that being overpaid? Never; and most aren´t paid that much.
Cheers