Vanilla tea fortnum and mason

Seems ok BUT the flavourings may contain sweetners or other chemicals and synthetics. Test, and test again.
Have you tried adding a drop of real vanilla extract to ordinary tea?
I have a nice cinnamon and ginger rooibos tea, from BIRD and something co, https://birdandblendtea.com/uk_en/gingerbread-chai.html
In the summer Asda did a very nice rhubarb and custard herb tea but not online currently

 

You can search by ingredient on birdandblend.
This one sounds nice
https://birdandblendtea.com/uk_en/fairytale-of-new-york-150g-tin.html

 

The link you provided includes the Nutritional Information.
This states there is 0g carbs and 0g of sugar as well as 0g far and 0g of pretty much anything.
Therefore it is unlikely to raise your BG. However, some find caffeine can have an impact. The only way to find out is to try it and test.

One thing you will hear/read again a and again is “we are all different”. The only way to find out what the impact is on YOUR BG is for YOU to tes.

 

I drink Twinings Green Tea, cranberry flavour and their Cranberry and raspberry. I think they do a cinnamon one