Tesco does sell tea with mint in it. I believe peppermint works as well, but most of the studies have been spearmint. I drink only straight spearmint tea, with nothing else in it (except a little sugar).
I order it online. Specifically, I ordered my current stash from here. But I think I'll order from somewhere in the US for my next shipment, because the orders take too long to ship from Ontario to Kentucky. In the UK, you can order from here. But I'm sure there are several places. That's just the first that came up when I searched.
In my experience, loose leaf is a lot cheaper for quantity than bags. 100g (3.5oz) lasts me almost a month having 2-5 cups a day.
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