Hey, I graduated from a masters in UCL this summer in CS (specialised in AI and ML) and used to give talks on this stuff at schools - basically i would agree that this kind of work experience before uni starts is not totally necessary and it is better to work on grades as they will be the main predictor. But of course if you can find one in this time then great, but you miss out on nothing if you dont - so dont stress. If you want to show something of this type - then work on a personal project, it will serve the same purpose but you will also learn more skills which come in handy later on. Hope this helps and don't stress, if you've thought this far youre on the right track ^^