It seems she was:Interesting solution... you'd have to come up with a way for this to be acceptable to the shipping companies, however. Remember that the channel is one of the most heavily-trafficked waterways on Earth, and in the late 19th century was even more so. With three enormous artificial islands blocking things up, shipping traffic is going to be massively affected. There's a reason no one tried a bridge in OTL -- it would have affected shipping too much.
With artificial islands, you've also got to deal with the problems of current eroding your 45m-tall (average depth in the Dover Strait) islands, as well as the problems that would accompany placing the iron rings with enough precision in the strong current and cold water. Incidentally, the sheer size of the rings would enable you to use the Great Eastern in the construction process, if you can imagine a way for it to still be available in the late 19th century.
By 1872 the Great Eastern had been made obsolete by purpose-built cable-laying ships. She spent the next 12 years laid up at Milford Haven on the south-west coast of Wales.
Just sitting there.