The other difference...
It appears the author, and a majority of the people making comments here have missed the other difference.
Leaving the "unlimited" argument out of it.
The £7.50 bolt on is only valid for use when browsing using your mobile phone, NOT for use with a computer as a bluetooth modem, or a datacard.
The £30 bolt on, covers usage via all means.
Quoted from o2 website:
"O2 Web Bolt On gives you unlimited* browsing through your phone for £7.50 per month
O2 Web Max gives you unlimited* browsing with a data card for £30 per month"
This is also confirmed by the conversation I just had with select customer services when I signed up to the £7.50 bolt on about 30 seconds ago.