Duty free must be bought at time of international departure, so at LHR in your example and not at MAN. However the MAN duty free store is happy to sell you the same goods, but for a higher price, for a non-duty free eligible departure (though that is hardly worth buying). With the elimination of duty free for many flights within the EU, the "duty free" stores have managed to finesse it, implying that they are cheaper and somehow tax-advantaged, even when they are not.