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If you want a bit of tea and cake, the Kew Garden cafe near the station is okay.
The food available in the gardens themselves is not bad, but it's pretty expensive. If the weather is nice, I suggest bringing your own picnic.
Edit: They have opened a new "proper" service restaurant called The Botanical now as well. I've only had breakfast there, but it was good. You'd probably need to book in advance for this one.
Going to Kew Gardens tomorrow, anywhere near by recommended?