Are Different Pay Rises for the Same Job Permissible?

I work in a profession that has a Chartered Institute. Membership costs a few hundred pounds annually but it is not a requirement to carry out the job. The membership process is through application that proves prior training and experience, there is no examination.
Some Companies will pay the membership fee for those staff that join the institute. The benefit to the Company is that they can say x% of their staff are members of the Chartered Institute. However, my current employer has a different system and I wondered as to its legality. Basically it awards different pay rises to staff based upon their membership of the Chartered Institute.
In April of last year the pay rise was 1% for non members and 3% for members. This year it was year 5% to non-members and 6.5% to members. There was no other reason for the differential and it does not directly or indirectly discriminate as per protected characteristics of the Equal Pay Act 2010.
The differential far exceeds the cost of membership fee to the chartered institute.
In all other regards the person standing next to me is as equally qualified and experienced as I am, yet because of their membership of a chartered institute they have beeb given a greater pay rise than me, a risk that amounts to more than the membership fee.
The easy solution would be for me to join the same chartered institute but, in the meantime, is this difference in pay awards based upon membership of the chartered institute permissible in law?
Replies
Accordingly, permissible in law.
Depending upon the profession, it is possible that some Clients may pay a higher rate for Chartered members than non-chartered, or may do so for certain activities, or may specify that certain activities can only be signed off by a Chartered member. All of these scenarios would mean that the Chartered member has genuine added value to the employer that a non-Chartered member does not. This would support justification of the salary differential. Supply and demand may support the justification of the variable pay awards meaning that the salary differential is increasing.
It is quite common to need to demonstrate SQEP (Suitably Qualified & Experienced Person). The Q is demonstrated by academic certificates or training courses. The E is quite often demonstrated by Chartership.
Experience is not simply time in a role or time working following completion of structured academic / vocational training (though training is never truly completed). Does someone who has been in a role for 10 years have 10-years experience, or one year's experience ten times? Chartership will be assessing the breadth of that experience that 10 years experience really does mean 10 years experience, not one year repeated.
Chartership applications, while not having an exam, normally require more than simply paying the fee and are quite challenging to demonstrate the compliance with the necessary criteria. As the OP has the experience, I wonder why they don't choose the easy solution:
If the employer suddenly decided to pay any member of staff who had a degree double the salary that would be lawful too!
As has been suggested, you virtually answered your own question. Basically an employer can "discriminate" in any way they like, except for a handful of characteristics prohibited by law (e.g race, gender, religion etc).
Certainly in my industry having qualifications from the chartered institute can help you demand a higher initial salary when getting a job. I've known some who give a small uplift for staff when they become qualified. Neither of those directly considered it in pay rises but when you have a higher starting position and add 3% then in absolute terms you're getting more of a pay rise.
To be honest though, at the point of you being qualified (well before chartered) then its the norm you are on individual salary and individual pay rises rather than blanket