Yep, anything leafy. Google is your friend, here! White, green, black - assam, oolong, lapsang souchong, darjeeling, rooibos (redbush = same thing...) etc. Also mint and nettle. Clipper apparently do a dandelion one which appears to be only leaf, but not seen it to read the full label. Fennel could be either leaf or seeds (or both) and I'm actually not clear what LT's stance is on seeds. I suspect they're not allowed as they probably have carbs...
You do have to be careful to read the ingredients - there's a "raspberry leaf" tea that I got excited about, but then it turns out to have apple pieces and raspberry "flavouring" (which could be juice extract or something?) in it too, so that's a no-no! I recall seeing a lemongrass one too, that turned out to have added lemon...
If you have any herbs growing in your garden/pots, you could try making your own infusion just with ripped up leaves and hot water - I do that with mint sometimes (my Mum grows both peppermint and spearmint) and that makes for a bit of variety. I've always intended to get some lemon-scented thyme to try, but I've never got round to it!
Have fun,
Marianne x