Your wife's iPhone is now locked to Virgin, and they charge £15.32 to unlock it. Carphone Warehouse have been selling iPhones like this for ages, the devil is in the detail i.e it is sim free (until the first sim is inserted then it locks to that network) this is not the same as factory unlocked.
Details about unlocking different networks are here:-
https://www.uswitch.com/mobiles/guid...mobile-phones/
Apparently you have to call Virgin Mobile to request an unlock as they do not have an online service.(how quaint)
Dave