Hurrah! Congratulations! It's even better on a 3G or 3.5G phone, but 2.5G is perfectly fine for most things.

Sounds like 3's tariffs are ludicrously restrictive. Why the heck isn't there one that lets you use the connection for your phone AND a computer? Are they expecting you to buy two SIM cards and keep swapping them? Crazy 3. There's absolutely no technical reason for those restrictions at all, as you've found out with your old T-Mobile SIM.