If you want this job enough the 10k is a no-brainer.
This is your personal experience, and it may be true for many. However, the overall economics question for NATS is 'will sufficient people of a sufficient quality want the job enough to apply on these terms'?
As has been said, people who
really want the job will make whatever arrangements are necessary - saving hard before starting, taking out a loan, borrowing from family, surviving on their spouse's income, or just plain sucking it up. But are there enough of these people? I suspect NATS doesn't think so, which is why they are pushing the misleading £85k figure for all it's worth in the recruiting bumf - to attract people who are financially motivated by big numbers instead of aviation enthusiasts.
There is a middle ground, though, between the extremes of the spotter who will willingly flagellate themself to be given a shot at their dream job and the chancer who happens to see the £85k figure in an ad in the back of FHM, and I expect that that middle ground is where the best candidates are. A percentage of these high-quality people that NATS wants have other options that don't require them to impoverish themselves and their family to prove their worth. Perhaps it should be kept in mind that these people are applying for a professional career, not trying to join a monastery.