You could've chosen USD as a currency. As of this writing, the standalone download costs £13.74, 17.34 €, or $19.95. It isn't made immediately clear, but while BMT Micro provide a long list of currencies to choose from, only three are "
real"
: the USD, the euro and the pound. Everything else is provided for information purposes only, and the customer is actually billed in USD.
More importantly, it wasn't that good a deal, because the GOG version doesn't include the soundtrack, and it isn't sold outside of the $24.95 bundle. (And yes, the soundtrack by itself is worth the full price of the bundle.
)
As for the installer size, GOG bundled the .NET 3.5 distributable into their Windows installer.