The defence did consult medical experts, and a statistical expert. They chose to not use those experts in court.
Before the trial starts the prosecution and defence experts meet together to discuss the case. The barristers receive a transcript of that meeting.
Experts have a duty to the court. They can't give a biased or selective account to support one side.
So an expert could be asked to produce a report by the defence, but the expert's opinion not sufficiently support the defence,then they wouldn't be used in court.
The defence did have medical experts, so it wasn't that nobody wanted to be engaged by the defence team.