Though it's known as the "Global SIM," initially it'll only work in the US and the vast majority of Europe, including the UK (obviously). By year's end, however, FreedomPop is planning to expand roaming coverage to additional countries in Southeast Asia, Latin America and beyond. The SIM itself costs $10/£10 to buy and you'll need to add another couple of bucks/pounds to cover the shipping cost; but after that you get the basic service, which affords you 200MB of 3G/4G data each month to use across any of the supported countries, completely free.
Should you exhaust that allowance, you can simply buy additional data, which is one of the ways FreedomPop makes money. An extra 500MB can be had for $10/£10, or you can buy in bulk at discounted rates ($50 for 4GB, for example). International data roaming is obviously the main selling point of the SIM, which is why FreedomPop's also offering a WiFi hotspot to stick it in for the introductory price of $50/£39. You can just plug it straight into your tablet or smartphone of course, though you don't get any inclusive minutes or texts like you do with the provider's mobile-specific plans. Also, to use voice and text services by way of FreedomPop's mobile apps, you first have to buy a virtual number based in one of the supported countries.
Like the provider's mobile-specific plans, you will be able to "earn" extra data by completing surveys, downloading vouchers and the like. Similarly, FreedomPop's value-added services, like rollover data and additional virtual numbers, can be added to the Global SIM plan for small monthly fees. If all you need is the barebones service, though, stick within the 200MB monthly limit and it won't cost you a cent/penny.