First: if you already know this, sorry. Maybe it'll still be useful for someone else.

US and Imperial units are similar but not identical*. And where they diverge the most is also the most important to the field of cooking: volume. For instance, an Imperial pint is 20 ounces, but a US pint is only 16 . . . however, a US ounce is also slightly larger than an Imperial one, so an Imperial pint is actually about 19.6 US ounces. I totally ******ed up a quiche this way using an Imperial recipe.

Anyway, moral of the story is if you want to use a US cookbook with SI units, you want to convert US (not Imperial) volume units to SI. Likewise, if you want to use an English cookbook in the US, you'll still need to do unit conversion from Imperial to US. (Luckily, a teaspoon is 5ml everywhere.)

*Systems of measurement were standardized in both the US and UK at the dawn of the Industrial Revolution, and although they were both based on old English units, there were plenty of old units to choose from (which was of course the reason for standardization in the first place).