Honestly, I think you should wait this one out till your of age. Like others have said, your parents are only doing this with your best interests in mind.
Going by items you've picked up in the past it appears to me that they have no issue with you having Anime, just nothing that's unsuitable for your age and given that many people struggle with getting their parents to buy them a show like Bleach they should really get the benefit of doubt here (if you don't mind me saying so .. just trying to look at it in the long run, breaking their trust would could lead to them refusing to buy you Anime, etc etc)
Highschool DXD now has a UK license, you can get a debit card at 16 right? (I think?) so that's three/four years, the show will still be in print then (if not, there will be tons floating around, its a popular show) ..no need to rush :3
Do you have a Myanimelist.net or anything like that? Maybe we all could check out your watchlist and pick out a different harem/ecchi show for you to check out that isn't so .. full of plot