When you get a locked mobile... read on?

I'm getting a Samsung Galaxy S II and the cheapest I can find it is from 02, for �399, this I don't mind but my dad is persistently insistent on getting an unlocked handset and the cheapest I can find one for is �499! What's wrong with buying the one from 02 then getting unlock code? He tells me that whenever you unlock something something goes wrong with it and he was manager of phones4u before so I'm not sure... If there is a chance it will break what are the odds? Will insurance cover it? What will it cover as? Thanks!