Regarding currencies, some interesting facts:
Transactions in a given settlement period are always transacted in the currency of the marketplace they are related to. For example, a settlement from Amazon.fr will always be in EUROS. Settlements from Amazon.co.uk will always be in GBP.
Transactions have to be in the marketplace's country of origin currency in order for country of origin taxes to be applied properly. If your Amazon.fr settlements came into A2X according to your bank home currency of GBP, your system would apply tax rates according to the UK instead of the EU and your tax return would be incorrect.