Its one bath and a separate electric shower in another room but definitely won't be used at the same time. Maybe on the odd rare occasion.
30-35kw vs going for the 40kw will there be any advantages?
In my opinion unless you are going to be trying to run 2 taps at a time most people wouldnt notice they are getting 13/14ltrs a min compared to 16. Yes your bath may take longer to run by 2/3 minutes but every time you use a basin etc you wont be using it to it's full potential.
As you have said you are looking to extend my advice would be to go down the unvented cylinder route. Gets rid of you tanks in the loft gives you the best hot water.