You might want to think about what the "members of the public" are going to be doing with these PCs, and maybe say a bit about it (and why you think fast upstream is important).
For example if there is no practical method for them to store data or take it away (no removable media, no USB ports, no hard drive space), eg they are just emailing, surfing, and printing, you may find you don't need much data at all. If you start allowing iPlayer, YouTube, and other bandwidth-heavy stuff then the requirement goes up somewhat, but...
On the other hand if these PCs have the potential to be (ab)used as (for arguments sake) community P2P engines, you'll have some serious traffic management to think about.
Someone will want to make sure this kind of use is legal on the contract in question, and that the charitable organisation is suitably protected in the event of a user abusing the connection somehow.