Ah, Jim, it's all part and parcel of the iStock experience.
Check
this thread out. It was clearly stated that
no images from Kew Gardens would be accepted here. I wondered how they would know that a particular tree, flower etc had been taken there and was accused of trolling!
However, there are 90 photos here keyworded 'Kew Gardens', c40 uploaded since that thread, some of them featuring the distinctive buildings. Yup, I guess they might have got property releases. However the balance of likelihood is not; usually when I write enquiring about PRs I don't get a reply. The one and only reply (not Kew) I got quoted a price such as even if I sold 500 images (which wasn't at all likely) at an average of $2 each, I'd be quids out.
Nearly fell out with my sister two years in a row when we've been in London and I've refused to go out to Kew.