Sorry, but it is a scam. Unless you're from within the EU its almost impossible to get an employment Visa in the UK now without a highly skilled role available, offered to you, and with sponsorship from the employer.
A friend of mine recently had exactly the same issue and I checked out the address and called the contact.
UK telephone number, non UK ring tone, VERY african voice. Tried to convince me that his was an american travel agency with an office in england. The office address is a post office - Look up the address on GoogleMaps and streetview - you can often SEE the address they are claiming is NOT what it should be.
Pleeease don't send money. You won't see it again.
