HMRC has its own criminal prosecution policy which can be found here. Criminal tax investigations are time-consuming, resource intensive and costly, so HMRC selects its cases carefully. The main triggers are:
Materially false or forged documents or materially false statements have been made
Repeated tax offences
Wider criminality
Serious organised crime
Where an individual holds a position of trust or responsibility (such as a lawyer or accountant), or a well-known individual
VAT – missing trader intra-community (MTIC) fraud
VAT repayment fraud
Organised Tax Credit fraud