"How will we tell?"
"it seems only a matter of time before researchers, campaigners or plain old forgers start trying to get them through borders. And in the case of the latter, if they succeed, how will we tell?"
On behalf of the Ministry of Truth:
There is nothing to tell. It is not, as you suggest "a matter of time" before a compromise will occur, due to the stringent security measures in place. The UK government takes data protection and security very seriously, and has ensured that, by initiating a cross-comparison lookup biometric database, which will remain offline and therefore secure, and will only be used in exceptional circumstances, that 100% security will be maintained at all times by being tough on ensuring the validity of customers of UK Borders and Immigration and their safety.
Next question.
NEXT QUESTION!!!!