It used to be the case that you had to have two doors between, but it isn't anymore. You should probably check with your local council about building regulations anyway. I pity the Fooooo-l who relies on answerbank.
I think I am right in saying that if this situation already exists then you cannot be forced to change it, otherwise most of us would have to change our houses on a continuing basis as building regulations change. If you are building a new house obviously this would mean it would have to comply with current regulations.
PS I live next door to the local Senior Building Control Officer!
I thought that there had to be 2 doors. I live in a house that is approx 5 years old and there is 2 doors between the toilet and kitchen and I presumed that was why it was planned that way. However I recently went in my neighbours house for the first time, which is a slightly different design, but same age and there is only the one door between the toilet and kitchen there. So I presume it is not essential. Unless requirements have changed in the last 5 years.