Try Mimo's Cafe near Hyde Park, especially if you're looking for a good value. I got the Full English Breakfast in the photo for a mere four pounds fifty. The sausage (buried in the back) had a smooth filling that Jenny enjoyed, too. Aside from the quality, though, where else in London can you get full in a sit-down restaurant for less than eight dollars US?
I had this "breakfast" for dinner, by the way.