The main reason for the situation you describe is that Britain operates what is known as an �adversarial� justice system.
The founding principle of this system is firstly that defendants are innocent and it is the prosecution�s job to put before the tribunal (either a bench of magistrates or a jury) sufficient evidence to secure a conviction. The defendant does not have to say anything, but if he does not respond to the evidence provided by the prosecution, the tribunal has only one version of events on which to pass a judgement.
This is an admirable system but it has a number of shortcomings which sometimes means the truth remains hidden. The defendant cannot be forced to say anything at all, but if he does give evidence he can only be examined on that evidence � not on anything else. Skilful advocates manage to conceal the truth without the defendant actually having to tell lies.
Other countries operate an �inquisitorial� system where the judge or magistrate can ask questions of the defendant on anything he wishes, can draw assumptions from silence, and are more able to uncover the truth.
The difficulty with your proposition �...there�s almost no doubt that they are somehow involved� is arranging for that decision to be taken. As CORBYLOON says, who makes such a decision? Of course, the prosecutors believe the defendant was �somehow involved� or they would not be prosecuting. So they (or the police) are hardly best placed to decide.
The system we use is not perfect. There are sometimes acquittals of the guilty and sometimes convictions of the innocent. However, the sort of �pre-trial trial� you advocate is not really a step forward. Evidence can only be considered by an impartial tribunal, and our system already provides for that to be done.