It's a scam.
The "solicitors" are the same scammer or an accomplice, who plans to make you pay fees for this imaginary certificate.
In order to get an employment visa for almost any country (including UK) you will physically have to visit that country's embassy in your own country. If it is possible to get a visa for that job (UK has put a hold on most types of work visas, only allowing very specialized workers to get visas), you will do it through the UK government, and they are the only people you will pay fees to--and those fees will not be from Western Union.
The only legitimate website you need is the UK border agency website
http://www.ukba.homeoffice.gov.uk/