When the estate accounts are prepared all figures need to be converted to GBP.
So if you have a USD bank account it could have a description like "US Bank a/c 12345 ($50000 at death exchange rate 1.86)" but the base value in Probate Plus will be the GBP figure. Probate Plus does not hold the local currency value. For UK Capital Taxes, the figures are entered as their GBP values.
This extract is taken from the Inland Revenue IHT215 - practitioners guide : "You should show the value of the asset in it's foreign currency. Then, show how you have converted that value to £ sterling. Major currencies should generally be converted at the closing mid-point figure given in the "Pound Spot Forward against the Pound" table shown in the Financial Times. Less common currencies may be converted at the rates shown in the 'FT Guide to World Currencies' published weekly in the Financial Times on Mondays.".