According to reports, the Trent Bridge wicket will be a sporting wicket along with the Headingly one. If that is the case, I dont see the point in playing Giles. I personally wouldnt bother playing a spinner at Headingly unless he is world class (ie Vettori, Warne, Kumble, Mruali).
If Hoggard and Johnson are fit, and pass well through their county matches, then they should be considered. Or the other option is James Kirtley, I dont think England could do anything wrong in playing him.
I wouldnt pick him for ODIs, but Tests he might be worth a shot.
If you want spin, I'm sure Vaughan can bowl himself.