There is no such thing. I crafted this Google search (note the quotation marks).
The only hit is this question.
More broadly, the term "certificate of eligibility" is used for foreigners seeking to be students in Japan; British attorneys seeking certification to practice law; and US veterans seeking to use the VA home loan program.
The main point of Brexit is that UK is not desperately searching for foreigners to work in the UK and send their wages overseas.
However lots of dirtbags make money selling this belief to their own countrymen. They sell dubious "services" that are either outright scams, or are rather unlikely to fool the vigilant British.
It is illegal to go to the UK to seek work, unless you enter on a type of visa that is specifically for working. Those are hard to get. As such, people often misuse non-working visas, get caught, and get banned from the UK and other anglo countries with which the UK interchages data.