Although the Immigration Rules regarding Secure English Language Tests (SELTs) changed on 6 April 2015 it was not possible to take a UKVI-approved IELTS test until 23 April 2015 (and in some countries even later than this). Therefore results certificates of tests taken between these dates will not contain a UKVI number, even if they were taken at an approved test centre.
Since it was not possible to take a UKVI-approved test during this time the Home Office has agreed to accept tests taken at any IELTS test centre between 6 April and 22 April 2015 (inclusive) as a SELT for Tier 4 applications made before 6 November 2015. This includes tests taken at IELTS test centres that are not on the Home Office's list of approved centres.
The Home Office has told us that they will add this information to the gov.uk website shortly.