I tried a fair few firms back when I was at school: no luck so I gave up. I heard recently that a cousin who is in the same position I was when I tried got some: so it is the luck of the draw I think. I think Geldards (at Pride Park) are probably the best firm in Derby though - for commercial work that is, I don't know what you're interested in.
But I would say that if this is to put on UCAS forms to study law I wouldn't worry about it too much: but do try to sit in the public galleries at the crown court, that should be enough to show an interest in Law. I'm not saying give up, I'm just saying don't let it do your head in - no biggy if you can't get any.