How many days in London, Paris, Madrid (and barcelona?)


I am travelling with My husband and 2 teenagers at the ene of June. We have a total of 15 Whole days ( not including the half days that we travel) and are thinking of doing 5 days in London, 5 days in Paris and 5 days in Madrid. I really want to squish in Barcelona— maybe cutting London to 4 days, paris 4 days, 3 days in Barcelona and 4 days in Madrid. Or stick to the original plan and only do a day trip madrid- barcelona via the AVE? What are your thoughts?! Thanks in advance!!