There have been a number of posts on this site already about Umbrella Companies, but I’ve had an enquiry about employment rights for people engaged through such a company.
The Low Incomes Tax Reform Group has published a useful briefing [pdf] on this here. and there is also a web-page that explores this in the wider context of employment status here.
Also, one of the commercial facilitators of Umbrella Companies is CWC Solutions – their website is useful and informative.