I get the feeling that you are unconvinced.
£300 is half of £600
£306 is half of £612
So same as before. So fair.
Consider the following scenario.
Someone on £600 a week is considered to be worth twice as much to the company as someone on £300 a week. To be fair that should remain the same after years of inflation and wage rises.
At the start the more valued employee has additional income compared to the less valued one, that allows them to go on a second holiday, eat out once a week, go to the theatre every month. This is the accepted starting position as one employee is worth more to the company and reaps the benefit of that.
If a percentage is used, after x years of wage rises both might find their salary is 100% greater than at the start. The lower valued employee has £600 a week, but finds they can still only afford the same things as they used to at the start. Meanwhile the higher valued employee has £1200 a week, but also finds they can still only buy what they could at the start, which still allows them to go on a second holiday, eat out once a week, go to the theatre every month more than the lower valued employee can.
Both are still in the same situation as they always were. Which is fair. The figures may have changed but the affect is identical.
Now compare this to a different system where the same increase was awarded to all employees.
After x years The lower valued employee finds they have £600. A rise of £300 since the start. Their situation still hasn't changed. However the higher valued employee also only has a £300 increase so gets £900. Which means they have lost out as they can no longer afford the second holiday, and can only eat out once a month and visit the cinema four times a year more than the lower valued employee.
This is clearly unfair as their salary has dropped in real terms, but the company is still getting the same work out of them.
The result is either going to be that the company is swamped with applications for the less valued job positions or can find no one to apply for the higher valued job positions because rival firms attract those capable of filling those roles.
If you have an issue it is with the differentials not the pay rises.