My initial thought was that the effect is visible during the test period because the employees are especially eager to make this work shortterm.
As the period was quite long, there might be a real effect.
If they can’t rise wages then the only thing going would be more days off. The UK Government should do that first considering their pay wages are so low compare to other countries.
11th in terms of net PPP (just behind Germany, but ahead of France), and 6th in terms of net median monthly salary (unfortunately there's no PPP on the median chart, not that it'd make much difference given how close GBP and EUR are now).