Paying class 2 NI at about £150 per year gives access to a qualifying year for state pension. It has been possible to pay voluntary Class 2 NI where self-employed earnings are low (below the Class 2 small earnings exception).
This option will no longer be available for 2019-20 where S-E earnings are below £6,025.
In order to obtain a qualifying year Class 3 NI can be paid, which is exactly five times the cost of Class 2 NI.
The self-employed with low earnings might be tempted to omit some of their business expenses in order to clear the small earnings threshold and thus avoid having to purchase a qualifying year at five times the price.
This poses an interesting philosophical question as to whether omitting business expenses is non-compliance with tax rules. Also in terms of a tax audit, it is rather more difficult to search for understated vouchers rather than overstated vouchers. Does one ask to see the omitted vouchers folder?