IME of the voluntary sector, senior managers pay for staff drinks from their salaries because the company can’t use their (charitable) funds for this. £50k is a very high salary for a charity worker. Most staff are on much lower pay. So, senior managers pay as a gesture of goodwill to thank people for their hard work.
It seems OP doesn’t disagree with this approach and is happy to pay a share towards the drinks of 15 staff. It appears OP can afford to treat their staff. Indeed, to not do so would seem a bit mean spirited, when earning £50k.
OP’s problem seems to be the principle rather than the cost. Because of the vast difference in salaries between the Senior Management Team, with some on double the salary of OP. I assume this isn’t just the CEO. Surely this salary difference should be addressed in general if you all have similar responsibilities. Are the higher paid SMT members mainly men? Or perhaps OP’s role isn’t as responsible and therefore shouldn’t really be classified as SMT.
On the drinks, it seems reasonable for SMT to chip in according to salary, given some SMT members are on a vast salary: £100k. In fact, if there are only 15 staff, couldn’t the CEO pay for everyone’s drinks if earning £100k?
The issue is whether to suggest this to other SMT members. IMHO, that really depends on what relationships within SMT. Most charities believe in fighting for a fairer, more equitable society. That should surely start within themselves. So, I don’t see a problem with OP raising it at an SMT meeting, to ask if it’s been considered. That’s surely the sort of challenge to the status quo that’s needed from SMT to create a more an efficient and fair organisation. It should be accepted in that light.
But SMT in some charities can be quite entitled and tone deaf. There can even be quite a toxic atmosphere, where challenges to ways of working aren’t well-received. If that’s the case, OP would have to weigh up if it’s a hill they want to die on. It’s only once year. How much would they save? If less than £100, might be better to suck it up, rather than get colleagues backs up.