With most phones, when you are locked to a certain provider, you can use any sim of that provider. However, I'm getting "no service" with a UK iPhone. I've even heard that AT&T US iPhones allowed different sims of the same provider.

Should I be able to use my other o2 sim?

Also, when downgrading, I skipped using iBrickr to get past DFU mode. Could this have been an essential part in pairing my SIM with my phone? Is there any way to fix this?