Is your speed advertised as Up to 20MB, or exactly as 20MB?
If the way WiFi speeds are advertised wherever you are, are the same as they are advertised in the UK, The "Up To" bit is key. What it means is that if there's hardly anyone else using the line, then you're likely to get the 20MB as that's what the line is capable of.
If you try again, but during off peak hours, you may see your speed increased.