I thought Milan has plenty of shops with good bargains, especially for fashion? In what way is Germany/UK shopping better than Italy?
Milan has the brands, but they are the full price shops so there's no bargains. In fact, Milan feels more expensive than other Italian cities, but that's probably just the perception.
If you're talking about pricing, the Italian brands will tend to be cheaper in Italy than they would be in London, especially at the outlets.
If you're not talking about branded stuff, then you can get plenty of good stuff in UK - like M&S is probably about at least 30% cheaper than SG (and if you go in winter and buy out of season items like shorts, it might be on the discount rack where the discounts are huge) or there's also primark and other places. To be fair, you'd probably find these local type of places in Italy (or anywhere else in Europe) but I don't know of them.
As for the TS question of whether to go to UK or Italy, it really depends on what you want to do and what you want to see. A suggestion is to go read up on both countries and see what interests you more. If you are not confident and this is one of the first time you're travelling, then UK speaks English and it'll probably be easier for your first trip (although I've not had problems in Italy)