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.
+1, came to say this - pretty common ID request, easy to recognize.