No problem applying few days before expiry of visa. But new rule says you cant extend ur stay after expiry by 28 days. So better check that http://www.ukba.homeoffice.gov.uk/sitecontent/newsarticles/2012/june/53-overstayers And from experienec it takes ages for postal applications. I will recommend ...

When did u apply? Have you got any intimation from UKBA? Maybe u can reapply before ur expiry by getting a same day appointment. But u will need funds in ur name for 3 months. Maybe from a different account.