You need to use GiffGaffs 'Lost Phone Network Recovery Service', LPNRS for short.
GiffGaff have offices all over the country for just this kind of eventuality.
All you need to do is findone locally, take your phone there and tell them where you found it.
The offices are all known as 'Permant Offices Licenced Iphone Communicatons Environments'
(POLICE for short)
You can recognise them because the staff generally wear blue uniforms.
Now for the important bit...
You need to make it clear to the staff what you want to do:
Make sure you tell them :
1. You found the phone.
2. You have no intention of returning it to the rightful owner
3. You would like to know how to unlock the phone so that you can get it working again.
I am sure the staff would be more than happy to help you...
how do i get a network on a lost i phone 4s
Don't change the sim card. Report to the police, the phone can be traced if used and you could be accused of theft if you have tried to activate the phone for your own use. Do the right thing - be honest.