The UK uses an odd mixture of both depending on context.
The use of "100ml" in airports is because using "3.519 fl oz" would be confusing to far more people. Even within the UK we use metric for small liquid measures like this (smaller liquid measures end up being weird stuff like "teaspoons" or "tablespoons").
And this isn't just because the UK uses a different fluid ounce to the US (100ml is 3.519 UK fluid ounces and 3.3814 US fluid ounces).
Anyone under the age of about 60 in the UK would had metric measurements taught to them at school as it became a mandatory to teach it in 1974. Many schools would have been teaching it already, and probably lots since the currency changed in 1971 (https://en.wikipedia.org/wiki/Decimal_Day).
The youth of today (as seen through the lens of my kids) are very metric, often defaulting to distances in meters and kilometers. Miles only seem to be used idiomatically, e.g. "he lives a few miles away".
I'm completely happy to switch between all of them not just because of my UK education covered them all, but I've lived for more than a year in the US, the UK and some European countries.
There are still plenty of examples of mixed measurement systems in the UK though.
Canned/bottled drinks are marked in ml, but a lot of that is due to the proximity to the EU and the previous ties to it. Open drinks are often sold in imperial measures (pints, etc) although spirits moved from fractions of a gill (imperial) to metric (25ml for a single, or 50ml for a double) in the mid 80s.
Of course the UK and US pints are different sizes (568ml and 473.176ml). Not just because the fluid ounces are different sizes as noted above, but also because the UK has 20 fluid ounces in a pint and the US 16 (of its) fluid ounces in a US pint.
For driving distances and speeds are based on miles, but for pedestrian distances you'll see a mixture of miles/yards or km/meters. Restricted heights (e.g. low bridges) or widths are covered in both feet/inches and meters given the number of European freight drivers on the roads here.
Occasionally you'll see some nonsense where a sign has displays both, and where the actual distance to something might be shown as "400 yards" it had almost certainly been rounded up/down to that whole number to make it simpler on the sign, but when it is converted to meters the converted value is used, so you see odd things like:
"
Whatever it is
400 yards
365 meters
"
(The UK traditionally used "metre" but that usage is quite rare now and we've mostly moved over to using "meter" like the US does.)
I'm surprised that the UK and US don't have different length miles (the US did have a different length "foot" but the "Survey foot" was discontinued in 2023).
