VATREC12030 | Rounding on invoices and rounding at retailers: What do we mean by rounding up and down?
From HM Revenue & Customs · VAT Traders’ Records Manual
The rules for this are simple:
If the VAT on any transaction comes to less than 0.5 of one penny, it should be rounded down.
If the VAT comes to 0.5 of one penny or more, it should be rounded up.
This is the same system as that outlined in EU Council Regulation 1103/97, which sets out the rules for converting national currency values into Euro.
However, if a trader proposes an alternative rounding method then it should be considered and allowed if it produces a fair and reasonable result.