Jonathan Strange is a silly fop who becomes a popular hero then a tough cynic then.. well, something even more. I think the obvious and best casting for this role is Hugh Laurie. However I think there are a wide range of attractive british character actors who could carry it off a treat.
The fairy lord is a mad, immoral, wicked enchanter. I would love to see Jude Law in this role. I think he'd be frickin' excellent.
Here is a scene between Strange and the Fairy Lord - not too spoilery. Imagine Laurie and Law.
'It was your...' Strange paused to find the right phrase. '...kind attentions to the King of England that first brought you to my notice. Such power! Such inventiveness! English magic today lacks spirit! It lacks fire and energy! I cannot tell you how bored I am of the same dull spells to solve the same dull problems. The glimpse I had of your magic proved to me that it is quite different. You could surprize me. And I long to be surprized!'
The gentleman raised one perfect fairy eye-brow, as if he would not object in the least to surprizing Jonathan Strange.