The donor of the phone can probably get it unlocked before handing it
over. If it has been on a contract it is usually free. If it was
bought on PAYG, it will cost about £15. What network is it on?
They are usually locked to a network, not to a specific sim, so if it
was on Orange it won't need unlocking (although might be worth doing
anyway for future). If it is not on Orange, you might consider
switching your number to PAYG on the network that would work.
Once it will accept the SIM, it will just work. The only thing you may
have to manually change is that the data settings it installs
automatically will be for the special iPhone tariffs. If you change
those to the standard data settings for your network, it will use
whatever data options your sim has. More specific instructions depend
on the network.
Worth accepting even if you decide not to use it - worth at least £90
if you sell it to CEX!