If you have already taken an English language exam such as IELTS or TOEFL by the time you're applying then you can just include the results on your UCAS form (the overall score, the subscores, and the reference number so that universities can verify your results). Then all you have to do is take the original score sheet with you to registration or send it to the university in June (this is up to them so you'll have to ask... each university has its own rules).
If you're taking the exam after January 15 then you have to let the universities know as soon as you receive your scores. They may or may not want you to send a score report so that's something you have to ask once you have the results.
You have to let the university know of your exam results, no matter when you apply. I understand that you're applying while in your last year of high school, this means that your application will contain your predicted exam results and universities will make their decisions based on those predictions (and any actual results you may hold by then). Then when you receive your results in June you'll have to let the university know. They may want you send photocopies first and bring originals with you to registration (or they may want originals straight away... that's something you have to ask once you have your results).
The language test is important in the sense that you have to achieve at least the minimum they require (which may not be 6.5 for all universities and all courses!). A perfect score will be impressive, but it won't really help your application, all they want is for you to have reached a certain level of English, anything above that level is good and will help you in your studies, but it's not going to make up for poor exam predictions or anything like that (unless you want to study English or Literature or something in which case the results may have more weight).