Good question, aog ! We should try it. Nothing is going happen to us. Nobody interferes when we refuse entry to someone, or sends them back if they have got in briefly 'because their presence is not in the best interests of the United Kingdom', as the stock expression has it. That power is exercised peremptorily in cases where the 'presence' would present no threat to security . One example was an American TV show presenter who was convicted there of insider trading. Another, years ago, was of George Raft, an actor who was suspected of having Mafia connections.
Romas deported from France present a theoretical problem for France. If Romanian, thay have the right to move freely within the EU, so it's difficult to see how they can be prevented from returning, but I doubt whether Sarkozy cares much about that!