Remember the use of "Terrorism" Act against Labour Party protester?
"The problem is that no matter how much politicians promise not to use this against the "average" person it will end up being used that way."
This is correct. Regardless of whether or not claims as to legislation's *intended* use are made in good faith or not, experience has shown that this cannot- and must not- be relied upon.
One notorious example is the use of "anti-terrorism" legislation, specifically the Terrorism Act 2000 (introduced under Labour's watch), which was used against an 82-year-old German-Jewish émigré who had heckled Jack Straw at the 2005 Labour Party conference. Specifically, the law was (mis-)used to stop him getting back in:-
Regardless of whether or not one thinks he should have been allowed back in, the fact that a supposed "anti-terrorism" law was able to be used- and *was* used- against someone who clearly wasn't engaging in terrorist activity nor in a terrorist context shows that the law was badly designed (assuming it was designed in good faith) and that the very party who introduced it- and were still in power at the time(*)- couldn't be trusted to ensure that its usage was restricted only to the claimed targets. (**)
Similar arguments apply against the use of "anti-terrorist" legislation used to freeze Icelandic bank assets in the wake of the 2008 Icesave bankruptcy.
Whether or not one thinks action should have been taken against those respective parties, the fact that it was done using "terrorist" legislation is the concern, because neither were remotely "terrorist" and nothing like the targets the legislation was claimed to be aimed at.
A law that can be misused for something not remotely related to its claimed purpose (whether or not one thinks that a *specific* "misuse" is justified) is wide open to blatant abuse for a whole range of purposes, desirable or otherwise.
(This shouldn't be taken specifically as an anti-Labour rant; I despise the Tories, and didn't vote for them. However, many- myself included- assumed that they would (at least partially) stop and roll back Labour's egregious assault on civil liberties and pathological disregard for personal privacy. Instead, they're turning out to be just as bad in this respect).
(*) Whether or not it was the police's choice to misuse the legislation this way, the fact remains that Labour were the ones responsible for introducing legislation that could be misused in the first place.
(**) Of course, this assumes that the party that introduced the legislation remains in power to ensure its "correct" usage. Even if *they* can be trusted to act in good faith and ensure its correct use (and in the above cases, they obviously couldn't), this is irrelevant if and when they lose power.