At present all promotions have a mandatory requirement to be in 1 specific currency. This is frustrating as it then requires us to duplicate the promotion set-up unnecessarily.

Promotion set-up should be currency agnostic.

If I set up a % discount promotion - currency is completely irrelevant.
If I set up either a deal price or value off promotion - you could argue should the value being entered be in a specific currency and then converted to the currency of the transaction. I would suggest not. A buy 2 for 20 promotion should apply whether it is $20, £20 or 20EUR and apply the relevant discount amount accordingly. It is then up to the business to decide if they should make that promotion available in channels with different currencies.