Nothing, but you wouldn't know that reading archaeo-blogger Paul Barford's latest post about legal exports from the United Kingdom. Mr. Barford also intimates that export permits for coins are required, but under UK Export Guidance that is not so for most numismatic items in trade, i.e., published types not "straight from the ground." (Detector finds from the UK require export papers).
The UK is to be commended for a workable system of export controls and a PAS and Treasure Act that encourage artifacts found by the public to be properly recorded and retained where they of cultural significance. If only countries like Cyprus, Greece and Italy were as reasonable, they would likely not be facing many of the same problems they have at present.