You have a very good connection for the apparent length of your line, and you might be able to squeeze 1.5Mb or even 2Mb at a push out of it, if you upgraded to a Max-based service.
As havelock says, you're actually connected on a 1Mb line. Before MaxDSL, there were 4 choices of speed that ISPs could order from BT wholesale: 256k, 512k, 1Mb and 2Mb. ISPs have commonly provisioned a line on the fastest connection speed that BT say you can have - in your case it was obviously 1Mb, and then just throttle your speed down to 512k at the ISP end.
That way, if you wanted to upgrade to 1Mb in the future, they could just up the throttle without paying BT a regrade fee.
You'd have to go to their "Plan 2" to get upgraded to MaxDSL. Like I say, it might let you squeeze an extra 0.5 - 1Mb out of your line, if you think it's worth it.
What I would say is.. due to the length of your line, it would be best to make sure that your SNR is reasonably stable before upgrading.
There's a program
here called RouterStats which will graph your SNR margin and see if there are any major fluctuations e.g. at night, when other household equipment is turned on and off, etc..
Also, to improve your line quality, you can follow the advise in kitz's
Low SNR page, e.g. getting an ADSL faceplate, plugging your router direct into the master socket, etc.
Hope this helps