According to Google, the highest rate has been as high as 94%, in the period since 1945, but, as I said elsewhere, I never reached such earnings heights, in the relevant period, so, I don't recall it..
Another factor IMHO if talking of Income Tax is EE NI, this, to all intents and purposes is an income tax, in addition to what is referred to as Income Tax.
There is also of course the "indirect" taxes, very difficult to work out a meaningful figure, I suppose, but, if we take into account Insurance premium tax, council tax, fuel duty, VAT etc etc, what % of an average earners income goes to the Government?