The UK Border Agency, in the new Tier-4 points Based systems has not mentioned IELTS as one of the compulsory requirements but even then students are advised to show their proof of English in the form of IELTS/TOEFL, as this greatly helps in the visa, because it shows the sincerity of a students towards his/her studies.
When you apply visa with IELTS, it increases your chances to qualify for visa- up to 25%.
Students having O & A levels qualification are not required to show IELTS certificate but it is good to have it
So if you are serious to win a visa then you should take the test because you have to impress the ambassador for your visas so don't let any thing missing or you will lose visa, because the High Commission will not call you for the missing material. The only option they have is to refuse your visa.
Apply for your visa with IELTS certificate and maximize your chances.