The BT tester isn't fair, it comes from BT after all.
Those figures look pretty much spot on for a 1Mbps connection. Your d/s speed is slightly higher than I ever achieved, your u/s is slightly slower, but I doubt you will notice it. The variation in speed you are experiencing is probably down to your ISP, though might just be exchange congestion.
It might be worth your while, depending on how you stand contractually, looking at another ISP.