eliberate falsification of expenses is fraud. You would be committing a criminal offence and risking a criminal prosecution.
It is also a serious breach of trust and confidence, even if the amounts are small. Deliberate falsification of your expenses would almost certainly justify summary dismissal by the employer. Of course, the employer must still follow the Code of Practice on Disciplinary and Grievance Procedures (PDF, 1.63MB) when carrying out any dismissal.
Your employer must also follow its own disciplinary procedure if it is more thorough than the Acas Code (for example, if it contains extra stages of appeal).