Re: equal taxation
Now both can afford to live and to save for the future, so the state now has a stable £6 revenue.
That ignores the effect of indirect taxation.
Make the flat rate 10%, with 25% sales tax on luxuries. Cost of living is still £9/week.
A earns £10 a week and pays £1 tax. £9 to live on. Can't afford luxuries, but doesn't die, has incentive to earn more since even £1 extra will mean 90p to spend.
B earns £20 a week and pays £2 tax. £18 to live on. Spends £9 on essentials, chooses to spend £9 on "nice stuff". State gets 25% of that £9, £2.25. B also has incentive to work more, since they'll only 'lose' 10% of the extra.
Total state revenue is a stable £5.25. Even if B saves/invests some of their surplus any gains will be taxed, and it will be spent eventually.