tenancy agreement it should have at least six months before expiry.
you've got £15k in saving? then you could take the risk of applying, but its a bit of a gamble..
maybe better if you come to the uk on your own, find a job, get some pays slips as evidence that should increase your chances.
as for the job offer, is it a definate job offer ? becuase they may well contact the company.
you have to weigh up being apart for months or take the risk and go for it, know one can tell you your chances, but i do know people who have taken that risk and had savings similar to you and got their visa..
good luck with which ever you do thou
