I find it strange that three Heads of Security Agencies had to speak out for fear that Brexit would damage exchanges of information between France, Germany and the UK after Brexit. Why should it? They would have to want to change their current procedures, or their governments would have to stop instructing them to make sensible exchange.
It is already the case that if the UK gets intelligence about a threat to lives in France it will tell the French authorities and vice versa. There are data sharing agreements, based on what we can usually share with due consideration of how each Intelligence service protects its own sources. The UK belongs to the Five Eyes grouping of the USA, UK, Australia, New Zealand and Canada where trust is even stronger and the sharing has gone further, and that will clearly continue after Brexit.
This seems to me to be another non problem, unless the EU side wants to make it a problem. As we have high quality and extensive intelligence it is unlikely they will want to reduce the flow of information, so they can just agree to carry on. The information share is usually bilateral anyway. Issues in the UK should be adjudicated by our court, and issues on the continent by their court.
An Extradition Agreement might be a better route for bringing suspects to trial in another country rather than trying to continue with the Arrest Warrant, where ECJ jurisdiction would be a problem.