We're in the UK, and the soil was purchased from a UK supplier, but no telling where it's actually from, likely still the UK but who knows.
Largest cap is around 15mm across with a 25ish mm stalk. White all over except for the centre of the caps that have a light yellow spot.
Bit paranoid about eating any of the herbs from the planter now.
An ID would be great.
No need to worry about your herbs, it's just a mushroom growing on your potting soil and helping to break it down which may actually make nutrients more available for your plants.
Those photos are pretty blown out but from your description I bet the mushroom is the flowerpot parasol Leucocoprinus birnbaumi. They're pretty harmless though you shouldn't eat them or anything.
So I looked at the link from dandelion, and some other pictures online, and L. birnbaumi doesn't look close at all.
The ones growing in our planter are completely smooth. I'll try and get a better picture tomorrow, seeing as it's pretty dark any kind of lighting I've tried has resulted in equally or worse blow out of the pictures.
Thanks for the peace of mind in any case. I'll update with better pictures tomorrow afternoon.
they are L. birnbaumii or a close relative, there are no other look-alike mushrooms that are that size and color, and the fact it's growing in your potted plant is just confirmation.
The caps can be appear smooth, from the wikipedia article's description of the cap:
The surface is lemon yellow to sulphur yellow, smooth or slightly powdery and covered in ragged, fibrous scales which are easily removed.
The scales easily fall off and can leave the cap smooth.
You should google the species name and look at other amateur photos, you might see some of the variety that can be present and see some that look more like yours.
EDIT:
examples
