I don’t think there is anything legally preventing you from doing so.
However, see it from the perspective of an immigration officer. How can they be sure that once you’ve found a job you won’t start working right away?
Then you have the fact that if the job can wait a little while for you to start (usually more qualified jobs, where prospective employers are willing to wait for you to end your notice period with your previous employer, deal with visa issues, and usually with slightly longer processes, multiple phone and face-to-face interviews…), then there’s no reason for you to arrive in the UK first and look for jobs while there: you can do all the initial preparation (apply, go through phone interviews…) while outside of the UK, then plan face-to-face interviews and get to the UK for those.
On the other hand, jobs where you could find an opening quickly via a more direct door-to-door type approach are usually lower paid jobs, where you are expected to start quickly, definitely not with enough time to get back home, apply for a work visa, get one, move, etc.
In regular times, the chances of you slipping through the net would be quite large. Only if something tips them off and they start digging would there be an issue. But in this period where travel is often restricted, IOs will naturally be a little more inquisitive of the reasons you are coming to the UK, and once you say “I’m looking for work”, they won’t hear the “but I won’t be working on this visit, just looking for a job”.
If you’ve watched any of the “Border protection” shows, you must have seen that finding a pile of resumes in your bag, or a suit and tie when you pretend you’re on holiday, usually ends up badly.
Don’t forget the UK are flexing their muscles to try show that they are left with a shred of power after Brexit. You don’t want to get caught up in that. Not only is it highly inconvenient and uncomfortable on the spot, but it will in addition leave a stain on your record, so be extremely careful.