The UK would have deployed their Pykrete carriers to deal with the subs.
Besides, they nearly ran out of food even with the few subs the Germans had in the Atlantic for the first year of the war and those were smaller, shorter range subs. Tough to crew an iceberg or anything else when you can't feed the people required to run it and build aircraft to fly from it.
Very interesting idea, though. The Brits had a lot of those and some of them even worked.