Every online casino that I know of has a self-exclude feature so if you go into live chat or phone them and ask them to permanently close your account they will do it.  

One thing a lot of them do is ask if you'd like a "cool-off" period instead.  Don't choose that if you're seriously addicted; just tell them you have a gambling problem and you'd like a permanent closure.  All of the ethical sites will never open your account again.  I asked Paradise8 Casino to close my account maybe a year ago (?) permanently because I was too addicted to their slots and they did.  I tried to get them to re-open again a few months ago and they wouldn't do it because of my earlier request.  Reputable casinos and bingo halls will all do this.

It looks like you're in the UK because you play in pounds so a good site to get help is GamCare.  They have an online forum/chatroom, telephone hotlines, etc.  It's a great service.  I've put the link in the source.  Best of luck to you!