From the 2Mbps figure, and your "no problem" speed test, I would say you have been throttled to 2Mbps for exceeding the 300GB FUP. Have you downloaded (plus uploaded) that much?
That usually stays in force at busy times for 30 days. The 300GB restriction is supposed to be being removed in April, but whether (assuming you are being capped at 2Mbps) that would uncap you straight away, or whether the 30 days would have to elapse, I don't know.
(In case anyone queries the fact the BT speed test gives full speed, it seems to me fairly likely from previous tests that it bypasses the BT Infinity in-house routers so wouldn't hit the throttle).