My understanding of the Brit military is that it is a professional military rather than a volunteer military - by which distinction I mean that while most US troops sign up for limited stints, the Brits, both enlisted and officers, tend to view their service more as a career.
My understanding is based on "some thing I read somewhere sometime ago" which I believe was from a reliable-ish news source, but if anyone has real knowledge or experience to enlighten me further I would appreciate hearing it.