If you're hacking around with phones or GSM boards for fun, on and off, it is cost effective to just grab one for a pound or so every time you want to do something as they sometimes come with a tiny bit of free data, or just a number to receive SMS on at least.

If you don't top up (with say a tenner) within some months the card deactivates and becomes useless, so it's a no-commitment way to access the GSM network.