If you want to camp near a tarn, Stickle Tarn in the heart of the Langdale Pikes might be an option. Just follow the path at the back of th Stickle Barn pub.
Otherwise, Angle Tarn or Sprinkling Tarn might be options. Simplest way to get there would be to follow the Cumbria Way, from the Old Dungeon Ghyll for 1.75 miles then fork left at a footbridge and climb for another mile to reach the former, with the latter being a further 1.5 miles further on.
I will add the caveat that while I've seen tents at all of these spots, I've not camped at any of them personally so I'm not sure how ideal they'd be. They are all certainly picturesque & dramatic though.