E10 is not petrol containing 10% ethanol. It is petrol containing not less than 5.5% ethanol.
The old regs allowed up to 5% bioethanol - i.e. a maximum. It has been fairly well publicised that some of it didn't contain any, but it was labelled as regulation required it to be with 'E5' to show it did not contain more than 5%.
The amended regulations specify that premium petrol i.e. petrol with 95<=RON<=97, must have not less than 5.5% and not more than 10% bioethanol.
How much it contains will presumably depend on what is most convenient and or cheapest for the supplier. Recently ethanol has been more expensive than petrol so my guess is that unless the oil price rises significantly than the average ethanol content of premium will be nearer 5.5% than 10%.
I believe September is also the usual month for the switch from summer to winter petrol. The difference is that winter petrol has a higher vapour pressure and lower energy content. It's also typically cheaper. So oil companies may well be combining the switch to E10 with the move to winter fuel. It just so happens that adding ethanol increases the vapour pressure. But you can also increase VP with butane, which as far as I can make pout is cheaper than ethanol.
I expect there will be all sorts of blends around as there normally are. Some suppliers will be 'converting' summer petrol, some will be buying winter petrol, all will have to contrive >=5.5% ethanol by 1 November at the latest. So you might not even be getting 5.5% ethanol from 1 September.
assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/986376/draft-em-the-motor-fuel-composition-and-content-and-the-biofuel-labelling-amendment-no-2-regulations-2021.pdf
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E10 fuel new
E10 fuel new
