There is no guarantee that sea deployment for Apache will become common.
The airframe no matter what you do is not particularily weather resistant when it comes to salt water. There are plenty of areas where water can get in and stay. I would not advise it long term. As for upgrading the AH-64Ds -there is no reason why the work couldn't be done in the U.K under Boeing .
I suspect that the technological rights are not a massive issue as the parts of the upgrade that would most appeal to the U.K are on the mechanical side of the airframe.