A couple of clarifications:
--There is no such thing as an "international lock". The only thing there is (apart from Activation Lock) is a carrier/SIM lock.
--There are no Apple Stores in South Africa. Only resellers.
-- There is no such thing as an International SIM or European SIMs. The SIMS are carrier based, not country based.
You phone is carrier locked even though you seem to believe it is not. Apple does not unlock phones. Only the carriers do. Now, at what exact store was the phone purchased in the UK?