To answer these
1. you have to find out if the UK is using "brake HP" test results that is with just the engine with nothing attached to it such
as with a belt off crank for power steering, generator, etc, no full exhaust system, no tranny, etc
Today GM tests engine design output as to SAE process that uses an engine dyno, and some exhaust and measures and results are at the flywheel
SAE testing has to be done with an SAE engineer watching the tests, and multo engines have to be tested and then the average of those results as what
GM can report as to engine HP and Tq. GM began doing this with the start of C6 Z06s LS7
Europe due to higher SMOG rules adds crap to the exhaust system and costs maybe 25 flywheel HP less than here in USA
2. Your Corvette's engine was designed to use leaded gas, way before that crap Ethanol was forced on us and reduces HP and fuel mileage
and in your case will F up your valvetrain, carb, mufflers, etc
Best case is stay away from E10 (here in the USA they forced E15 being sold
If you cannot find leaded gas, use a lead additive or spend the money to have the heads reworked that have internals Ethanol
will not F up like the valves
More on what makes up SAE engine dyno testing standard check my forum
SAE Testing