Don’t agree, and neither does official UK guidance https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/915637/220_Alcohol_Back_Calculation_v2.0.pdf
back calculations absolutely do depend on what and when he claims to have imbibed.(which he obviously lied about)
alcohol level may continue to increase for between 30 minutes and 2 hours after consumption.
Chap stopped by police, scared , just had 2 pints, says he had a drink the night before. Clearly lied. Prosecution assumes he drank a lot more than he admits to the night before. Assumption, unproven.
Not denying it’s likely the prosecution is right but standard of proof is beyond reasonable doubt isn’t it?