Interesting, one of the first highly publicized Hoover-ville's of the modern age:

https://en.wikipedia.org/wiki/Hooverville

I want to know how these people are in the UK legally.

To get a work visa you need to be sponsored by an employer. I struggle to believe UberEats and Deliveroo are sponsoring people from Brazil to come to the UK.

Discussion on Reddit suggests there is account sharing happening. Somebody with the right to work signs up and somebody else does the deliveries.