Andrews told Wren he had until this Friday to comply.
"We urge you to seek immediate legal advice ... Sanitarium's Weet-Bix and Marmite brands are ... both extremely successful; they are iconic New Zealand brands."
But Wren said his shop had long run out of Marmite, and he sold only one or two packets of Weetabix a week.
"Who told you I have Marmite? I don't," he wrote back.
"I own a tiny shop struggling to survive providing a little comfort food for UK expats and do not sell food online nor do I advertise any of the products you are concerned with anywhere.
"As far as I am concerned, your employees invaded my little piece of England in an extremely smug manner and threatened me with legal action within 30 seconds. That, I find unacceptable."
Wren said he had found there were Kiwi shops in Britain selling products including Weet-Bix to expats.
"Leave me alone," he wrote. "You have already generated enough bad publicity for yourselves, don't make it worse ... I don't have any Weetabix left."
In 2010, Sanitarium ordered Weetabix off the shelves of the English Corner Shop in Auckland's Onehunga.
Yesterday, the company could not be reached for comment, but Sanitarium general manager Pierre van Heerden has previously defended the action against Wren.
He said it was "normal trade practice" to protect intellectual property, and it did not matter that English Bob's Emporium was a small boutique shop.
Wren told the Herald he planned to seek advice on whether his selling the products was illegal, but even if it was, he said, Sanitarium's demands were over the top. "Who do they think they are? The CIA?
"I'm going to get some more in and I'll just stick my own label on it."