There is a code of practice with regard to police powers of entry published by the government.
if you turn to paragraph 10 point it does say that the power should only be used and should be proportionate and only with authority unless it’s required urgently to protect life or property from harm.
Depending on the extent of the leak, that may be satisfied. If it was a drip, it is unlikely to be satisfied. If it was running water, then it is.
It is also likely, depending on the reason for the leak, that it is going to be your daughter who is going to be responsible for the damage and repairs to the property below , which could have been more serious had everyone waited for the plumber to arrive.
Until you know the extent of the leak which will be visible depending on the damage the downstairs flat, it’s impossible to say whether the police acted reasonably or not.