Using a PC and Google Chrome here, lets me put in the tick, my number etc, starts and then packs up....
'The Performance Tester is currently unable to run a speed test for your broadband connection. Please try again shortly, however if this problem persists, raise the issue with your service provider.'
I've had that problem a number of times. I have also had that it is busy as well so many times that I have now given up with it and raised a ticket with BT.