What was the relevance of the tea?

First of all, I don't think there is any significance to the fact that it was tea at all (let alone what brand of tea it actually was), only to the fact that it was "hers". The story just demanded for some more or less trivial secret that only Maggie knows about and that is only mentioned in passing (and seems rather irrelevant at first), and which Foyle later references to Maggie's (and the audience's) surprise. The tea in this case was nothing else than a mere plot-device with the purpose of showing that Foyle ventures far deeper into Maggie's privacy than she would like. Even if not too personal and haunting, her favourite tea is still a far more personal present (and thus shows far more dedication and intrusion from Foyle's side) than just a bouquet of flowers. While for this something else equally secret and trivial would also have sufficed, the tea served this well, given the rather limited environment in which Maggie and Foyle met and the thus rare opportunities for him (and us) to pick up something about her.

I agree that the aspect of Foyle stalking Maggie (which as written above was the sole purpose of the tea) wasn't followed through during the further story. But I wouldn't go as far as saying that it had no impact on the story at all. It certainly showed (both to the audience, and, even more important, to Maggie) what she maneuvered herself into when deciding to become Foyle's barrister. So she learned a similar lesson as Will did, even if not in such severe and tragic dimensions. And it yet again reinforces Foyle's threatening and malicious aura, even if that threat was rather empty in the end. Maybe Foyle didn't even want to intrude into Maggie's life as much as he did into Will's but just play some little mind tricks on her.

So to conclude, the sole purpose of the tea was indeed to show that Foyle was effectively stalking Maggie or at least playing some little mind games with her. And while this thread was not followed through to some more serious consequences, it still showed to Maggie that it wasn't without a bad by-taste for her to engage with Liam Foyle and that she too had reasons to reconsider her attitude towards bending the law for criminals as evil as him.

Maggie's tea of choice was the Crystal Flush Makaibari Silver Tips Imperial Darjeeling FGTFOB, which comes from the oldest tea estate in the Indian Himalaya in over 1500ft, plucked at the summer solstice at full moon. The supply is extremely small, hard to get by and the most expensive tea sold out of India.

It is a refined taste and seems to be the one single luxury that she treats herself. I believe it could not symbolise a more personal object representing the character of Maggie who does not give away much about herself except to close friends.

Hence Foyle knowing about the tea made it an even more intrusive and mind-boggling experience, completely overwhelming the character.