Save a data feed with different product id for Google base

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Save a data feed with different product id for Google baseHi, I have successfully created a data feed for the UK and see my products listed in google shopping ok. I was wanting to upload the data feed for additional countries, but according to google need to change the “unique ‘id’ attribute values” in the data feed to qualify.

Is there a way of being able to mass edit the data feed so it would put a prefix before each product id, denoting the target country and thereby differentiating the feed from that which already exists?

At the moment should i submit the data feed as it is, then it simply overwrites the existing feed, but applying the uploaded feed to the country last selected. The result of which is my list of products appears in the most recently chosen country and is deleted from the one before.
I hope this makes sense