Short answer: Yes it is prestigious.
Forbes just ranked the MBA program here 7th in the world and 3rd in the UK in its rankings of non-US 1-year MBA programs. Equally, the Financial Times has the course ranked 3rd in the world for value for money.
Accounting and Finance at undergrad is currently ranked 2nd in the country by the Times, etc etc etc. Essentially the management school here has an exceedingly good reputation. The physics dept. also receives similar sorts of praise.
As for your second question 'Is is "posh"?', no, not particularly. The vast majority of the undergrads here have been state school educated (if that's how you want to measure 'poshness').
So yes. It is a good university, and to refute the previous poster, there are an enormous amount of career prospects here across the subject spectrum. Certainly b/c of the strength of the management school businesses take a high level of interest in employing graduates. Equally, the strength of the research here means that outside academic institutions are v. interested in recruitment from the university. Over the past summer I obtained placement as an Intern at the US Library of Congress, due in large part to the reputation of the University both here in the UK and overseas (certainly the number of European and Asian students would also seem to imply that Lancaster has a high-profile international presence).