I have already mentioned Hugh Laurie.
My problem with Hugh is that if he wants to put on the American blues voice he really should make it clear that he is acting out the role of an old jazz/blues singer.
Maybe, he should have created a persona, and called himsef "Professor House", or something, but going out as Hugh Laurie and singing, like he does. is comfortable to watch.

As I have said, I really would like to see/hear a whole generation of UK pop/singers who perform in a clearly recognisable British accents.
It was understandable that, in the early days of rock, UK singers would really be heavily influenced by the accent of Elvis and so on, but, as the years ticked by, I wish British rock had found it's own voice rather than continue to sing in a mock US accents.

There have bben lots of discussion on the web about this subject, and some of the things said show how our ears deceive us.
For example, I've seen people producing lists of UK singers who sing with an American accent, and also lists of singers who don't and sound British, But, amazingly, the same names appear on both lists. Oasis is one that comes to mind.
    As to Plant and Marriott, they tend to sing "over the top" at times and I prefer a more "controlled" singer like Paul Rodgers who call sing in an intense way, but without the histrionics.