Firstly, I don't know where you got those numbers from. According to Fortune, 11,727 people have been tested, and 6.3% have been positive. Most of these are asymptomatic.
https://fortune.com/2020/03/27/coron...ovid-19-tests/
Secondly, my personal guess is that the virus was probably introduced into Iceland much later than into the U.K., simply because there are far more people traveling from China to the U.K. on an given day than from China to Iceland. Statistically speaking, it would make sense that the virus arrived in the U.K. several weeks ahead of Iceland.
Thirdly, both because the U.K. has a population density which is 80 times higher than in Iceland, and because the U.K. has more immigrant minority groups that have a far more intimate social lifestyle compared to a Nordic country like Iceland, it is reasonable to expect the virus to spread far quicker in the U.K. than in Iceland.
So, to sum it up: I believe that this number from Iceland could indeed strengthen the argument that maybe up to 50% of the U.K. population are already infected.
What Italy and Spain are doing isn't an indication of the spread of the virus. They Italian and Spanish governments are reacting to the fact that deaths are skyrocketing, but the people dying today were anyway infected before any restrictions were put in place.