I keep it in my spare mobile, so I don't use it that often.
For mobile broadband I bought a 3 sim from eBay that gives me 1GB a month for 12 months. That cost me something like £20.
I've also got an Orange sim from eBay that cost 99p and gives me 250MB of internet a month for a year. It also came with £5 credit. They're still available here:
http://www.ebay.co.uk/itm/Micro-SIM..._Home_Phones_Sim_Cards_CV&hash=item35c1422bd9
There are always loads of good deals like that on eBay, much better than paying the networks' going rates.
I didn't say they "pop-up".
When you're using the browser you will occasionally get a page of ads instead of the page you requested. There's a link at the bottom of the ads to the page you wanted. There's no delay on this so you can just immediately click through to your page and ignore the ads.
This may have changed since I last used it though, their website used to explain how the ads work but they now seem to be very vague about it.