MPACUK - MI5 Lying

Just a couple of days after the IHRC declared that the number of terrorists stated by the head of MI5 was an "exaggeration", MPACUK has joined the party. In an article they claim that MI5 made up the figures and released them to enable the introduction of ID cards.

They first state that, "We have the Head of MI5 giving us scare stories of hundreds of terrorist cells operating in the UK."

Later on in the article they states that 7/7 was the only real terrorist plot in Britain:

He will definitely remember 7/7 as this is the only real terrorist plot with real casualties.

Why do these groups think they know better than MI5? Do they actually know exactly how many terrorists there are out there or do they think that they can continue this denial indefinitely?