Your GO6000 should comfortably hold maps for much more than just the UK, but not all of Western Europe.
I was always under the impression that the 6000 got you your entire regional map set (i.e., Western Europe) and that the 6100 got you the 'world' (any map you could possibly want). That said, you may not be able to download the full Western Europe map without a memory card. Those maps no longer quite fit into the 8GB memory of your 6000.
So I'm not clear about your specific issue. If you believe your 6000 is 'owed' the full Europe map set, or if it was installed on your unit before, and you just can't download it now due to size, a 16GB Class 10 memory card would be the right move. If you're saying that your device only comes with and is only provided with UK&ROI for free, that's another story. Here in the U.S. the equivalent model (without cellular) came with the entire North America map.