The judges forgot to point out in their very diplomatic language that it is also 'unclear' that there were any co-conspirators in Dhiren's, er, conspiracy: the 'Al-Quaeda' chaps he is said to have contacted told him to get lost and stop bothering them, as I understand it.
Violent terrorist and master criminal Dhiren Barot, known as the 'doodle bomber', has had his 40+ years' sentence cut to a mere 30+:
So: his plan wasn't really detailed enough to be called a plan; he didn't try to carry out this plan/daydream; even if he had tried, it is unlikely he could have executed it; and in any case even if he had managed that, it wouldn't have actually worked. Apparently, setting fire to some gas canisters in a vast draughty carpark, even assuming it could be done at all when they are inside a limo rather than on top of a huge bonfire, might not make much impact on a massive concrete building. Who would have thought it?
So he is being banged up for a 'conspiracy' which consists of: fevered scribblings in his Paperchase notebook; a silly video complete with playground 'explosion' sound effect consisting of a feeble throaty sound; and some sort of abortive attempt to get in touch with some people who may have been 'terror suspects' in their own right, but certainly didn't want anything to do with him.
The judge at first instance certainly isn't going to win any understatement prizes - he described this deluded loser who had never managed to so much as break a window as a 'very accomplished terrorist'!
Still, since the US were going to extradite (i.e. disappear) him once the UK penal system had finished with him, I suppose he had little sensible option but to plead guilty over here - assuming that he was rational enough to even make that decision.
Prosecution case against al-Qaeda Briton
[EDIT: this is a good detailed summary of the case.]