Yes, I agree with you. Besides the Alhambra, there simply isn't that much to do in Granada. Like you said, Barcelona would be a better pick. I went there once too and I enjoyed it a lot. Probably more than I enjoyed Granada.
Aw, you can easily go visit Granada right? I mean, if you're from the UK then it's easily accessible, right? But seeing as you're so excited about it, I'm pretty sure you'll have a great time there. If you ever visit Granada, check out a hammam --> an Arabic bathhouse. You can completely relax and feel like a new person after having visited