Odd - It wouldn't occur to me to convert to a single unit: -
VAT on one Pound = four Shillings; exactly
VAT on one Shilling = 2.4 Pennies, rounded up. so One Shilling -> 3 Pennies, two Shillings -> five Pennies, three Shillings -> eight Pennies, four Shillings -> ten Pennies
Multiple of five Shillings the VAT is a Shilling (exactly)
So for £17 13s 4d
£17, the VAT is 17 x 4 = 68 shillings = £3 8s
13 shillings = ( 2 x 5) + 3, the VAT is 2 shillings and 8d
4 d, VAT is less than a penny
£3 8s 0d + £0 2s 8d ==> £3 10s 8d
I guess if I was doing it every day then the "68s = £3 8s" would become automatic
I remained convinced for several years that my parents had swizzled me when my pocket money went from 6d to 5p in 1971!