Mobile internet providers?

OK I have a spare BT phone line which is costing £12/month and atm is connected to an ISP for my spare DSL connect in case my main one goes down.
So its costing me £27 each month total to just sit there waiting...

I have several mobile phones inc a T-Mobile Pulse Android phone, none of which get used very much and all on payg cards.

I am thinking that maybe the money I fed BT and my ISP each month can be better used by a sim that I can use for calls and txt with mobile broadband capabilities?
I would want to be able to slot the sim into my laptop on the odd occasion my main ISP at home goes tits up, or even use the Pulse on broadband if away from home and feel the need.

What are my options? What do you use?

If you've got an iPhone you can get a cheap broadband bolt-on, allowing you to use it as mobile broadband. I think orange offer a similar deal too

If I need to use broadband on the go, I just connect to my iPhone via Bluetooth/Cable and use that.