Can we legally fill a larger bottle to a smaller stated volume if the label clearly specifies the actual contents?

Yes, this is legal provided the label clearly states the actual volume (e.g., 70cl) regardless of the physical bottle size. Members confirmed that major spirits brands do this regularly — Del Maguey have used this approach for years in the UK, and US tequila companies commonly fill larger bottles to smaller stated volumes. The key is transparent labelling; as long as the label specifies 70cl, you're compliant with trading standards. In practice, fill-level visibility won't be an issue, especially with ceramic bottles where the interior isn't easily visible.