I generally recommend:
https://www.preplounge.com/en/mental-math.php
for general mental maths practice and
Speed Distance Time | OASC: RAF Officer and Aircrew Selection Centre
for speed/distance/time/fuel questions. One caveat about this website is that all of the questions use "nice" numbers which can get you unstuck if you get thrown questions where you are forced to round or estimate answers.
If you are anyway programming inclined it's fairly simple to make your own questions with more variety and difficulty.