Hello All
As I understand it, here in the UK even our cheaper fuel brands are much better (octane wise) than those found in some parts of the world, even the USA, so any bike designed to cope with the range of fuels found across the world will run just fine on UK E10.
However, modern petrol is nasty stuff, is far more agressive and goes off far quicker than fuel did say 15 or 20 years ago. The modifications made to vehicles to accomodate E10 are largely to offset the affect to rubber and plastic components in the fuel system, but E10 still absorbs more water so is still a problem to those of us with metal tanks and particularly vehicles likely to be left standing for periods unused.
I'm coming from recently using 40+ year old Japanese bikes on carburettors, so using E10 was not an option. I've been seeking out Esso supreme 99 (reputed to have the least ethanol content) plus using an additive (Wynn's Specialist E10 Protector) but even then, over the last few years the bike started suffering carb icing in cold weather (possibly due to 'wet' fuel), so I switched to Silkolene Pro FST which fixed that, but at 47mpg + additives, not cheap!
So then, with the 350 meteor at almost double the mpg, using E5 is a small price to pay, and I'll still add some additive if I end up laying it up over winter.
Some retailers are a rip-off with E5 at 20p extra a litre, I've found the local Tescos seem to offer a similar product to the esso at about 6p a litre extra and similarly Sainsbury's don't charge much of a premium either.