Since this was before 911 - the 'terrorists' in question were presumably the IRA, not sure they had much of an online presence back then.
You do know that 9/11 was not the first terrorist attack on the US by Islamic extremists, do you?
The fact that their plans would be written in a foreign language (never mind a foreign alphabet) would have been more than adequate to keep it secret from British intelligence.
ps. You do know that we have been having terrorist attacks for almost a century.
This is one of the stupidest statements I have ever seen.
Plus, in my experience, Arabic is a difficult language for people who grew up speaking romance languages to learn. Perhaps not as difficult as Japanese, but still much more difficult than German or French.
That's probably where they get them.
We may have a stifling bureaucracy and an overstretched military, but at least the British Foreign Office actually has local knowledge and people who speak the language.