Generally, write it out the same way you would for a non-picture book story. Don't break it into the pages you think it should be arranged when published, don't comment on where the pictures go or what illustrations are needed unless you need to make something clear (for example, you might need an illustration showing exactly the opposite of what the story seems to imply; say, a young girl tells her mom that her room is clean, the illustration needs to show a room that's a complete mess).
There can be exceptions to this, of course, but this is the generally accepted method of submitting a PB story.
This is the US "standard". I assume it would be similar in the UK but I'm not sure.