Managers seem to think that just because they say something their employees believe it when the reality is the employee doesn't care and will just replace the faulty manager.
So I think the only good source of market rate is people (either candidates or current employees asking for counteroffers), and if you're poor at being competitive for recruits on all axes other than money, or at making it safe for employees to interview and tell you about their offers, you have poor information.
That hardest part is figuring out which generic categories match our specific positions, since every company in the survey has different definitions and requirements.
Our employees seem pretty happy with the results, and we've had very few offers rejected for salary range reasons. So it seems like we're pretty close. If anything, we're probably a bit above our local market.
I used to work for a company that used their data. The reports they provided did not have any technical positions included. There was no transparency on their process.
Comparing those numbers to the Bureau of Labor Statistics, the salaries for anyone doing technical work were below market rate. https://www.bls.gov/bls/blswage.htm
BLS data is transparently provided and very specific.
I worked at a place which got the salary data, and then announce that everyone was underpaid and that salaries would be adjusted 'soon'. When is 'soon'? Why, it's inaudible mumbling.
Managers not MBAs is a good book that summarizes the problems with the current management culture.
Basically, by having a class of people who have never done the grunt work of a particular job, you ruin what the Army calls "Unit Cohesion".
The UK was no different to the US.