My theory is England doesn't have an equivalent to the CW/ABC Family etc. That's not to say there aren't young shows like Misfits etc. but there isn't an economy of churning out young actors to feed a system like CW.
Instead of moving out to LA at 18 and auditioning for the remake of 90210, British actors study acting at college and university and do stage plays till they hit about 22/23, then try and get in the game.
Take John Boyega for example. I just Googled "John Boyega stage play" and it pops up that he's gonna be doing a run in British theatre this/next year.
Could you imagine Michael B. Jordan leaving Hollywood money behind for 6 months to a year to do a play?
I don't think British actors are better, but they train more, and for longer, before they get seen by the world. That's why by the time you first see Elba or Oyelowo etc. they're in their early 30s and not 21.