You can only do it one month before.

http://www.direct.gov.uk/Topics/Moto...263&chk=8FPCGa


An MOT can be carried out at any timeIf the vehicle is presented for test within the calendar month prior to the date that the MOT is due, the test certificate will run from the date of the test to one year after the expiry date of the current certificate for example if the current certificate was due to expire on 1 April 2003, and the vehicle was presented on or after 2 March 2003, the certificate would run from the test date until 1 April 2004.

To have this extension, the vehicle owner should present their old certificate to the test station to verify that the extra time is applicable. The tester would then write the serial number of the old certificate in the appropriate section of the new certificate.

The same rule would also apply if the vehicle was taken in within a calendar month of its first MOT. In this case, the vehicle owner would need to present their registration document and the serial number from this document will be entered in the 'last certificate serial number' box on the MOT.

If the vehicle is tested earlier than one month before it's due date, it will only run for twelve months.