Thank you - just to clear up the matter. This has not been at the airport / train station. Just a central London / touristy spot.
A note has been made of their place of birth (obvs outside of UK) so perhaps that would somewhat make their "faux pas" more understandable.
Also could you please explain what an "unspent criminal conviction is" ??
The Rehabilitation of Offenders Act 1974 means that less serious criminal convictions become what is known as 'spent' after a certain period of time. A 'spent' conviction does not need to be declared for most purposes (e.g. insurance, job applications), as it is considered that the offender has received sufficient punishment for their offence after their offence has become 'spent'.
A conviction under the Railway Byelaws is considered 'spent' immediately. A conviction under the more serious Regulation of Railways Act 1889 is not considered spent until 1 year after you have been convicted.
There still remain some applications and the like for which a conviction that is 'spent' must still be declared. This includes high level security clearance, detailed background checks for working in sensitive occupations (e.g. police, financial adviser), and certain visa or other immigration applications.
Unfortunately I don't think anyone here is a qualified immigration adviser, and as a result
it would be unlawful for us to offer you immigration advice: if you require any such advice, it would be advisable to
find a suitable adviser. (Even if it were not unlawful, with matters that are as significant as immigration, relying on internet advice is not a good idea!)