Now, let's take a moment to examine that poster, shall we? The Doctor seems to have changed his outfit again, something he's done a lot more frequently as the Eleventh Doctor and for Eleven it's always a clue towards the current state of his psyche (or used to indicate whether stories take place in the past or future, when it involves his bowtie). And Clara doesn't exactly look as terrified as I would to be riding on the back of a motorbike driven by a madman miles above the streets of London with gigantic shards of glass all around.
Those shards interest me more than anything else in the image. I'm sure you've noticed they bear images of various monsters, including what looks to be a Dalek. Possibly this is related to the Wi-Fi angle of the episode, with the digital and physical worlds crossing over in some form. Maybe that's the Doctor riding out of the internet and into London itself. Or maybe it doesn't accurately represent anything in the episode and just looks cool.
Oh, and that title? The Bells of St John? If that sounds familiar to you, that may be because it's part of a nursery rhyme of the same name, and was used as a recurring motif in George Orwell's 1984, which was also set in London and had technology being used for nefarious purposes.. It lists off the names of churches with bells in London, and the line that talks about St John's reads " 'Pokers and Tongs' say the Bells of St John's". No idea if that's relevant, though. Perhaps its the entire rhyme or some other specific line that holds the significance. One of the lines does refer to the "Bells of Bow", which sounds like...
Yeah, I don't buy that idea either, but wouldn't it be so cool?