The government site got their info direct from the manufacturers (although they've done a thoroughly half-arsed job, allowing utterly inconsistent responses from some manufacturers and just not bothered at all to challenge 'sorry we don't know' BS replies, but that aside...)
Honda gave a very clear response and here it is straight from the horses mouth;
https://www.honda.co.uk/cars/owners/e10-fuel.html
They've given 2 answers because they know everything they've sold since 1996 has PGM-FI, whereas before then, some cars were PGM-FI and some were still carb.
So E10 is fine in 4th and 5th gens and in fuel injected 3g and 2g (2g and 3g are pre-1996, but the fuel injected versions are still PGM-FI).
1g and carb 2g / 3g are jobby out of luck and have to run on super, now.
As for the outlook going forward... Much less worried about whether still able to get E5, than general viability of buying petrol.
Personally, I think petrol use will start to become a problem, long before the 2030 ban of all ICE powered cars - on a cost basis.
If you think about it - no mainstream car manufacturer is launching pure ICE models now - they're all hybrid, as a minimum (and I can't see any mainstream car manufacturer starting development on any new model that isn't pure EV, now?)
Hybrids use less fuel - so we're already in the position where petrol use will start to rapidly decline, from today forward.
Petrol is expensive to produce and distribute, with massive fixed overhead costs (production/storage plant and distribution network) - the only reason it's as 'cheap' as it currently is ('cheap' in relative terms) is because of the high volume. Once that volume goes down, the fixed costs have to spread over much lower use - price per L will have to go up massively, to compensate.
So yes, petrol will still be available - but will it still be affordable, in the not too distant future?
Then there's the land value in the petrol stations - think about where most of the them are (in town/village centres). Huge land value. Once petrol volumes start dropping off, even the footfall to the retail store (where they generate a lot of their revenue) does as well.
At that point, why sit on a valuable asset that you can't generate a good income from, while housing demand is sky-rocketing?