SCAM - there is NO such company registered in the UK and that letter is an obvious scam written by a non-native English speaker
The address they gave you belongs to an industrial cleaning company called New Brush Cleaning Contract Services http://directory.leicestermercury.co.uk/...
There is NO job - it's a scam to get you to pay for a job that does not exist. There is NO company in the UK that can sponsor you for a job without proof they or their licensed recruitment agency in your country has interviewed you IN PERSON, verified your educational credentials and checked your references. If you have never been for an interview at their office in the UK, their office in your country or their recruitment agency's office in your country, any job offer will always be a scam. Also money is NEVER paid upfront for fees. The ONLY fees to be paid are the visa fees which are paid IN PERSON when you go to the British Embassy/High Commission for your visa appointment to be interviewed, fingerprinted and photographed. You don't pay before that and don't pay anyone but the Embassy/High Commission -- or their official visa agency which will be listed on the Embassy's website
Not to mention NO domestic job pays £3000/month plus a weekly allowance. Live-in help jobs pay about £300-400/month if accommodation and food is included. And there are NO visas for these type of jobs as they are not highly skilled jobs requiring a Masters or PhD that qualify for a work visa. Those type of jobs go to people already in the UK or EU citizens from Eastern Europe who do not require a visa.