Because the right to a jury trial is fundamental to our freedom from tyrants and oppression.
The Criminal Justice Act 2003 removed one of our fundamental rights, the right to be tried by a jury, and allows a judge to decide to dispense with the jury if there are allegations of jury tampering. This must be repealed and the right to jury trial restored.
It is easy to see how dangerous this is. Imagine the authorities believe they will have difficulty convincing a jury of someone's guilt and think they will have a better chance with just a judge. All they have to do is arrange, secretly and unofficially of course, for someone to phone one of the jurors and threaten them and as soon as the juror reports this, the judge can order a jury-less trial.
I appreciate that jury intimidation can be a problem, so if there are no better ideas, perhaps in cases where it's felt necessary, we could keep the jurors identities secret, by having them behind a one-way screen in court and escorted to and from court in windowless vehicles. This might not be ideal, but at least it preserves the right to trial by jury, which many people much wiser than myself have said is absolutely vital to freedom, such as Lord Devlin who described it as "the lamp that shows freedom lives".