Remove the ability for the police to use the magic words "I consider you a threat under the terrorism act" to be able to detain anyone they see fit.
A very clear example being the recent issues faced by 16 year old photographer:
@ 5:28 in the following video: "You know what, I consider you a threat under the terrorism act"
Why does this idea matter?
Although the example given may be trivial, it is only trivial until a particular group, race, religion or viewpoint comes under the crosshairs of the police force.
The terrorism act gives far too much power, with far too few safety measures to prevent the abuse of the act.
(I would ask for a larger review of all the legislation passed as a result of any terrorist attack during the rein of the last government. They appeared to be using the events as cover to sneak in large swathes of very dangerous laws. However, as I am no expert, I merely raise one issue I know about!)