What do you mean no option to buy additional storage? I use the legacy free product, too, and each user on my domain has (EDIT: had) the option to buy additional storage for themselves.

If I recall correctly, I pay 15.99GBP/year for an additional 100GB.

EDIT: hmm it seems that the page where I initially chose my 100GB plan no longer offers new plans. It just shows how much storage I have, and has a little info hover which says my storage is managed by my admin.

But in the help for the admin console, the page that describes buying additional storage says it's only for the higher plans. And another page says "As of June 2022, the G Suite legacy storage add-ons are no longer available to purchase."

So perhaps the old way (but storage per user) is no longer available, and the new way (buy in the admin console) isn't available (yet or perhaps ever).