I live in the UK.
Work for the immigration authorities - UKBA.
One of my roles is to ensure companies such as you mention can not operate in the UK!!
They promise much but can deliver nothing other than refer you to prospective employers who hold a licence.
Umbrella companies are no longer able to sponsor work permits this has been outlawed since 27 November 2008.
For Sponsor register
http://www.ukba.homeoffice.gov.uk/sitec ... ofsponsors
Unless the company holds a PBS licence they can not employ an OSN.
There is one exception to the rule Tier1 general visa holders.
This assume you are solely coming to the UK for employment.
For tier 1 which may be a better way see here
http://www.ukba.homeoffice.gov.uk/worki ... 1/general/
Also note if a company is not listed with OISC they are not authorised to give immigration advice in the UK see link
http://www.oisc.gov.uk/
Hope that helps.