Both the paid and free versions of Aldiko are in the UK app store...
http://www.amazon.co.uk/Aldiko-Limit.../dp/B004V1XJ6Q
http://www.amazon.co.uk/Aldiko-Limit.../dp/B004VMVF5C
The only way I know of to buy/install a paid "incompatible" app from Amazon is to buy & install it on another (non-Amazon) Android device. Then backup the apk and sideload it to the Fire. At least that's how I got Moon+ Pro and Mantano Premium on my HDX 8.9. I don't know if something like Bluestacks will work for this in place of a non-Amazon device, but it might.