Great question — answering as the UphillTech team:
You'll certainly be allowed to get on a plane in the UK with the tea, but you'll have to declare it to Customs and may have it examined by an agricultural specialist, who will decide whether it is allowed in. Here's US Customs and Border Protection on importing tea : There are no restrictions on the importation of coffee, tea or spices for personal use, although they may be subject to Animal and Plant Health Inspection Service (APHIS) and Food and Drug Administration (FDA) inspections if a Customs and Border Protection (CBP) Officer suspects contamination or insect infestation of the product. For more information from FDA call 1-888-723-3366. Contact FDA. Please note that tea or spices...
Full answer by Zach Lipton: https://travel.stackexchange.com/a/67365 (CC BY-SA 3.0).
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