Every 500MB of UK data will allow you to (approx):
 Send 500,000 emails, without attachments
 or send 1,000 emails, with photos attached
 or visit 5,000 simple sites, like Twitter or your favourite blog
 or visit 1,500 rich content sites, like bbc.co.uk, guardian.co.uk or yahoo.co.uk
 or download 50 (low quality) or 12 (high quality) music tracks
 or watch 60 YouTube videos, about 4.5 minutes long

If you're using lots of data and getting close to your network data limit, we'll let you know with a text message. And if you find that you're using a lot more 3G data, you can choose to add an optional Data Bolt On.

Don't forget, if you choose a Web Bolt On with unlimited wifi, you get access to BT Openzone at thousands of hotspots in the UK. And when you use wifi, it doesn't count towards your 3G data allowance. Use as much as you like, no extra charge.