Decanter is often associated with covering on high-end wines. However, they also evaluate supermarket wines, which can be found in the ‘Weekday Wines’ in their magazine. A tip is to go to decanter.com and select the Wine Reviews… Supermarket Wines section. Alternatively, this link currently works.
Any wine with a rating above 89 matches the quality of those high-end wines regularly featured in Decanter. They typically refresh their reviews twice a year: after Spring tastings and post-Autumn/Winter, following recent harvests.