Could you please post your broadband problems on the appropriate forum.
I read post is it not about mobile and lack of data
With all due respect, if you knowingly downloaded satnav material (which can easily reach 2GB or more) you were asking for trouble, irrespective of the broadband issue. I can't see BT going out of their way to help with that, nor would any other provider.
If you need help with the BB issue you'd do better to post info on the appropriate board.
If I understand your problem correctly you are saying you were using your phone to download via wireless on your homehub but because the wireless signal dropped out your phone switched to using 3g. As a result of this your 3g data limit has been exceeded.
If you had your phone only set to download on wireless it would appear to be a problem with your phone for either not stopping the data download when the wireless signal stopped or for reconnecting via 3g. It could be however that the phone is designed to do the switching once a download has started so that any important downloads are not interrupted and the data lost.
The problem you will have is determining if it is meant to do that or not.
I don't think BT will be able to be held responsible for the wireless signal dropping out because they, along with all ISPs will not guarantee a wireless signal because of the variables involved that can interfere with the signal.
gg has explained my point precisely. Your issue is down to the phone, its settings, or how you've operated it. It's a good idea to keep Mobile Data switched off until you need it, especially if you're doing anything heavy, just as it's a good idea to keep Roaming switched off when you're abroad.