That 600k will likely include some cars that probably should be taken off the road. Besides, owners will still be able to fuel them with your E5 premium. In the US, even that is E10 in the markets that require E10.
You will lose some fuel economy, but sounds like the UK is already at E5, so it will be a small loss. Most people probably won't notice it. Those that do pay attention will likely note the loss isn't worth buying premium octane to prevent.
Most of the old jalopies that are being run into the ground could probably continue to use E10. Rust will probably get them before the ethanol does. A fair proportion of the 600,000 is probably old cars requiring lead additives.
2006 Honda Jazz 1.2i-DSi S Vivid Blue Pearl
I pay more for higher octane which is staying at E5 anyway by the sounds of it, but I always favour performance over economy as I cover very few miles. I filled up with E10 because I was running on fumes, bit it wasn't out of choice.