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.
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 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