Learn English – Is the idiom ‘keeping well’ recognized only in British English

When I learned this “rule” (in first grade, I believe), it was explained that and separates the whole part from the fractional part: 2⅔=two and two thirds. The word and would only represent the decimal point in decimal numbers when they are read out in the formal “fractional” reading of decimals, as 2.3=two and three tenths, or 1.75=one and seventy-five one-hundredths. That is, according to this rule, *one hundred and fifty is ungrammatical because, if it is supposed to mean 150, it should be one hundred fifty, and if it is supposed to be mean 100.50, it should be one hundred and fifty one-hundredths. The rationale behind the rule is that you should only have one and in a phrase, so if the number were 403⅞, you wouldn’t say four hundred and three and seven eighths.

Of course, most of the time the decimal point is read as point: 2.3=two point three; 1.75=one point seven five or one point seventy-five; 100.50=one hundred point five zero, one hundred point five oh, or one hundred point fifty. The fractional reading of decimal numbers also starts to become a bit ridiculous if there are more than three digits after the decimal point: nobody would say 3.14159265=three and fourteen million one hundred fifty-nine thousand two hundred sixty-five one hundred millionths.

As you have undoubtedly observed, many Americans don’t follow the rule about and only being used to separate whole and fractional parts, and insert and just before the units of a number less than one hundred, although the forms without and are quite common too.

457 four hundred fifty-seven or four hundred and fifty-seven
2001 two thousand one or two thousand and one
1,000,001 one million one or one million and one

There is Fowler's Modern English Usage for British English. It is much longer than The Elements of Style, however.

Though I have never used it personally, you may want to check out The Complete Plain Words. That appears to be a much more concise, pamphlet-style guide to British English.

As far as your second question goes, I don't know of anybody (either in the US or UK) that thinks it is better to make your writing more difficult for readers. I probably cite this essay once a week on EL&U, but Orwell's Politics and the English Language is a small 'British' style guide in itself. Orwell puts forth several stylistic rules, or rather things to avoid in order to make ones writing more clear. I only bring it up because Orwell is British and he puts forth a great argument for clarity and simplicity in writing.

Fowler too advocated for clear writing (see the section on 'Approach'):

Henry W. Fowler’s general approach to English usage was to encourage a direct, vigorous writing style, and to oppose all artificiality — firmly advising against unnecessarily convoluted sentence construction and the use of foreign words and phrases and archaisms. He opposed all pedantry, and notably ridiculed artificial grammar rules not warranted by natural English usage...