I have a IPhone 5 new unactivated, will any nano sim work in it from the start. UK?

I'm on contract with Orange and have received my free upgrade IPhone 5 new in sealed box. Does this mean the phone is locked to Orange. I have heard that if i use another service providers sim to activate it, that network will also work. Any advise appreciated.UK only.

I'm pretty sure most Orange phones are locked to the network and you would need to get it unlocked to use other providers.