The UK is metrifing. But it takes time to make these shifts. Most hospitals will ask for weight in kg but usually have a calculator available. Charts and information leaflets have kg. Lift weight limits are in kg (usually then represend by people) Baby's weight is given in grams but usually the next question is what's that in old money. So the systems are all in kg but mentally we are in stones.

Doesnt the UK use a mix of both? Same as Canada?

US Customary Units and Imperial Units are not 100% the same, so three former the American system makes sense.

Liquids as in water specifically and not other liquids.

“Better still learn mental math, you won’t always have an answer book in your pocket!” — 1500’s teachers

In the olden days sciencey types and engineers kept fact books listing such relations. They had a specific name that escapes me.

Likely a "CRC": CRC Standard Mathematical Tables, which is probably the same book as the pre-calculated mathematical operations you mentioned. It has a lot of good mathematical stuff in it such as basic formulas, trig functions, log tables, integral and differential functions, and formulas up through around Fourier analysis.

If we were doing that thought exercise where you can take a "go bag" back in time to build science up more quickly, a CRC and a slide rule would probably be in mine. I'd probably throw in a prism and as compact of a microscope that uses a mirror as a light source as I could get away with; both for showing small stuff living in water and for showing what couldn't be manufactured yet (depending on when I went back). I wouldn't waste space in the bag for anything dealing with the imperial system.