Yes, they are allowed to keep your details unless you specifically withdraw permission - there is a legitimate business interest as you are their customer and they may need to contact you about your account. However, you may withdraw permission for them to contact you about anything else and there should be instructions on how to opt out. You may still get messages deemed legitimate business interests, such as notifications about service interruptions in your area. As to the messages, they will be generic and are probably going to all the account holders they have contact details for. It's cheaper than sending letters! And the Southern Water area us under severe water stress right now. Incidentally, in the UK it's illegal to cut off a domestic water supply, no matter what.