If you continue studying A-level videos, and maybe some textbooks - there are plenty available on Amazon and other websites - then you should be quite capable of studying Essential Maths 1 by October. The standard start expects you to have mathematical skills equivalent to someone who has studied at least AS level successfully and therefore even reasonable skill at A-level should be enough.
Studying at the level of the course could certainly make it easier but may also make it somewhat redundant for you, as you'll already understand many of its concepts at the same level they're being taught at. This isn't necessarily a bad thing, and should ensure you get a good grade, but may make it feel like you're doing the same thing twice over.
The previous maths courses were most likely replaced in order to keep their presentation and content up to date, in addition to merging some of them together. They also added the Introducing Statistics course recently which may be related to this. Many courses are replaced with newer versions every little while - you'll notice this in the prospectuses, especially the Computing & IT one.
I'd recommend getting a prospectus from the OU website. You can have them emailed as a PDF or sent to you by post, and I'd recommend getting the one on Maths(or any other subject you're certain you want to study) by post as it is far easier to read and check that way.
If you don't feel confident to study Essential Maths 1 by the time you choose your modules then don't worry about it. The gentle start still includes all of the Maths modules you need: Essential Maths 1 & 2 and Introducing Statistics. You'll cover everything you need to be capable of level 2 Maths regardless of the route you take - the main difference is whether you study Discovering Maths or a 30 credit module in another subject.
Good luck with your studies!