[niklasrde]

So updating the rules is not the same as updating the software? Who says this is different here? How do you define the difference between a "tax rule" and business logic?

The changes go beyond tax band thresholds and percentages across NIC & IT. Eg the (now-scrapped) Health & Social Care Levy would've been a completely new tax - you need a complex schema to encode that and even then the Treasury will come up with something you haven't thought of next.