Thats a sim breaker, also known by names such as "rebel sim" or "turbo sim" and sold by company's like "anynetwork"

The reports on these are widely varied - some say they have no problems, some say they simply don't work. Some get full use of the phone, but no data connectivity, some get everything, but sporadically on and off, and some say they work fine, then all of a sudden stop working... I must admit, even tho it's the cheapest option to buy an offline UK phone and stick a sim hacker in it, the risk is it could end up being a stupidly expensive paperweight.........