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BT (and SamKnows') estimator usually takes into account the quality and length of the line, as well as neighbours' speeds and previous speeds on your line.

In general I find BT's estimator to be a conservative underestimate, so if it is estimating 4.5mbps then I would expect around 5-6mbps in practice. 0.6mbps is certainly bad and suggests a fault somewhere seperate to simply the length and quality of your line.

There's two primary issues that affect your speed. Line attenuation (how much the signal gets weakened by) and noise/interference. Both of these values on your line seem much higher than expected.

BT's estimate puts your line length at about 3.5km, but your attenuation suggests its 4.5-5km. So your line attenuation (loss) is much higher than normal. Secondly, the noise margin on your line suggests you're getting far more noise ingress than is normal, as even with a 4.5-5km line and 65db attenuation, you should still see 2-3mbps not 0.6.

Both of these can be caused by a bad line, but more often by a bad connection somewhere, excess cabling, poor internal extensions, etc.

Even if you don't have a master socket I'd suggest connecting as close as you can to where the BT cable comes in, and disconnecting all additional extensions, the ring wire, and anything else hanging off your telephone circuit. That's as much as you can do really, as the rest is up to BT. If that doesn't clear the issue you've got a really bad or faulty line, which I'd check against the neighbour's to see if it's specific to you.