I've been doing a small amount of work on the side through these for a while (since several years before the pandemic), in a different area of work from you (writing and research mainly, with a bit of translating and voice works).
I did get a few long-term clients in the end, but it is very hard work, especially in the beginning. I've also used more traditional approaches, like working through lists of companies on LinkedIn, as well as various directories in my area of work and approaching "cold", plus keeping my eyes open for short-term contracts on indeed etc, as well as through other branches of my own employer. (All declared and legal.)
Since the pandemic there is definitely less work and more freelancers on both PeoplePerHour and Upwork than before 2020; and sometimes client expectations are beyond ludicrous these days - in "build me an all singing website for $50 in an hour" territory. You also need to take into account that the platform charges you what's effectively a 20% commission so that needs to be taken into account when you set your rates, along with tax, NI etc. At the start my rates just about allowed me to break even, but they have improved as I've gone on - not every client wants the cheapest rates.
Having said that, if you do sign up to Upwork, they have pretty good support for those new to the platform, or they used to - some of the seminars are very good and gave me some excellent ideas to improve my profile.
I wouldn't necessarily agree with the advice about setting your rates lower than every other freelancer (this isn't Amazon marketplace...), though you might have to compromise a little at the start. You can also limit the requests you see to UK only on UpWork; this approach has sometimes been effective, sometimes not.
I've also found for me, personally, the most productive approach is not to attempt bidding on any projects older than 24 hours (48 at the outside) or that have more than 15 bidders at most (preferably fewer than 10). You'll get really fast at skimming the offers and working out the scams or the ones where the clients will be an absolute nightmare to work with.
I'd highly recommend joining the Facebook group the Freelance Lifestylers, it's packed with kind and sensible people with great advice. There are others out there but I've found this one the best match for me.
Good luck, however you decide to move forward.