For those unaware it may be worth pointing out that the UK constitution has a principle of parliamentary sovereignty [1] - basically parliament can pass/amend/remove any law and no current law can bind future parliaments (they can always amend/remove it). Fundamentally if they say it's legal it is, so rules can be changed at any time such that GCHQ isn't technically breaking any rules.

[1] http://www.parliament.uk/about/how/sovereignty/