They don't. Any E10 fuel has to be labelled as such. I have only seen an E10 labelled pump in France or Belgium.
Standard fuel can be up to 5% ethanol without any mention of this on the label. The exact % will vary from day to day, depending on where your fuel was refined which could be the UK or Kuwait as it is a global commodity. A tanker may deliver 5% ethanol this week and 0% next week, or any ratio in between.
The alcohol actually raises the octane rating which is no guarantee of quality itself. The quality comes from the consistency of the fuel meeting the specification, cleanliness of the forecourt bunker, fuel age and the additive package.
Consumers seem to be locked into the false notion that a 99 RON fuel is always better than 95 RON when the most likely cause of poor running is that the fuel is full of shit that blocks your filter - don't fill up if you see a tanker delivering at a site as the shit in the bottom of the bunker will have had a bloody good stir. Equally, don't buy fuel at a rural station that has fuel delivered once every 6 months as many of those nice volatiles will have fucked off into the atmosphere or oxidised.