OK, what is the reason that got it barred in the UK?
You didn't mention that or at least I didn't see it.
Probably claimed as lost or stolen. I buy phones from many people and i sell them. One from ten gets barred. I don't ask where they come from. Maybe someone just sold it and said to their insurance company they lost it so they blocked the original phone and gave a new one to them.
At least you're honest.
As far as I know you can't get a blocked iPhone unblocked in the UK once its been reported as lost or stolen because the IMEI number goes on a barred database with all carriers.
It's then virtually impossible to get it unbarred unless you're the 'original owner' and have found your lost or stolen phone.