I had personally noticed a drop in economy over Feburary/March/April using E10. I read that although advertised as E10 for sometime, this is actually the maximum allowed ethanol content, rather that actual. I have a sneaky feeling that some fuels have progressively moved closer to 10% over the last year or so, rather than from day 0. I have exclusively been using E5 fuel from either Tesco Momentum or Shell V-power for 3 or 4 months and have noticed a 10%+ increase in fuel economy as a result so will be sticking with it. Looking elsewhere online the Tesco Momentum gets really good write-ups as does Shell.
Its worth noting that there aren't a million refineries knocking about so I can imagine that a lot of different suppliers get fuel from the same place (perhaps with slightly different additive blends). Fuel quality generally is pretty good nowadays and cars so well engineered that I cant imagine any of the fuels on sale in the UK would be "bad" for a car, otherwise OEMs would have issues with customers using it!