If you're sending it through your school/college, then you should consider how long the "checking" processes take - i.e. how long it takes for the admin staff and other people in charge of checking your UCAS application (to the extent of perfection) to finish and finally get an application 'sent off'.
At my college, it apparently takes 1-2 weeks to do (yeah, seems pretty long), which is why we have an earlier, internal deadline of September 26th to get our UCAS applications finished (to the best of our ability).
Otherwise the final deadline is October 15th, like people have said. It doesn't particularly matter when your application is received by admissions tutors - just as long as it's sent off to UCAS before the 15th.