Your choice entirely Baz.
He could just mark which rooms are the kitchen or utility rooms are. Then, you would go around with the kitchen designer, plumber, electrician, separately and set the positions.
That way gives you most flexibility, and the chance to change your mind.
If you leave it to the architect (and many do) you will have to decide beforehand where everything is going. That's no problem, but you will have to make sure he knows your thoughts.
The advantage here is that the builder will know from the start where the drains need to be etc.
Although layouts often change during the work, the most common way is to let the architect draw it all, so at least you have a starting point.
Don't worry too much. A decent builder and tradesmen are quite used to altering things if needs be.