That looks like the ISP has put you onto a fixed speed half Meg service.
Do o2 offer their full LLU range at your exchange, or is this O2 Access product?
The estimate fromhttp://www.coolwebhome.co.uk/calc/calculator.php is
Normal speed range at 58dB attenuation is 1850Kbps to 3000Kbps
Given your very good noise margin, if the ISP allow the connection to run at higher speeds there is no reason to suspect that a 3 Meg connection, and maybe 4 Meg is possible.