It has been explained that the 0.15% will be imposed by way of an adjustment to the exchange rate. If the spot rate is $1 = £0.72 you will get the same as if the exchange rate were 0.9985 x 0.72. So nothing different than what already happens when computing dividends. In my experience, dividends are rounded up or down to the nearest £0.01 when they are reported.
However, I recall that in the past Trading 212 were keeping records internally to more than 2 decimal places. So three dividends of £0.116 could be reported as three of 12p, but since those actually add up to £0.348, the free funds balance would show an increase of 35p, not 3x12=36p. This discrepancy of 1p was bothering people, so things may be done differently now.