IELTS general training option is the one they ask for. They have stated that as long as your IELTS is no older than one year at the time you apply, they will consider it valid throughout your application. That is, if your application takes so long that your IELTS becomes 2 years old, they will not request you to re-take it.
Of course you can do whatever you want. If you don't want to take IELTS, you can write a letter to prove your English proficiency, give them documents from schools and/or universities, letters from your professors or employers etc. You can send them all kinds of other tests like Cambridge proficiency and/or TOEFL etc. However, if you do this, there is no set score to points conversion and they can basically give you as few or many points as they like. If you want to be sure of your points, take the IELTS before you apply and you will know exactly how many points you will get.