But surely you'd want to write 3 shillings at £0.03, not as £0.15, wouldn't you?
Not at all - why on earth would you use a decimal place? I guess it might be because you are stuck in the 'I have this hammer, called the decimal point, and so everything must be a nail and use it' - hence why you faun over the idea that 335p can be changed to £3.35 so easily - it really doesn't matter.
And what's easier to add up in your head (the only place where the additional calculation matters):
25, 40, 55, 60, 20, 35, 85, 15
or
5, 8, 11, 12, 4, 7, 17, 3
The latter, surely, with the smaller numbers is slightly easier rather than needlessly having everything 5 times bigger? And obviously this would be far more annoying to add: 0.25, 0.40, 0.55, 0.60, 0.20, 0.35, 0.85, 0.15
It doesn't really matter that the answer is 67s rather than £3 7s, just as it doesn't matter for 335p or £3.35. The 20 times table is really easy and so you can as easily think "3 pound coins is 60s, plus 4 plus 2 plus 1 is 67s" instead of "335p is 3 pounds and 35p. 3 pound coins is £3, a 20p, a 10 and 5p is 35p". It's all a much of a muchness maths-wise. The only way it's harder is if you have learnt hundreds, tens and units and moving the decimal place, but not your 20 times table.