Rizla
Upstream is synced at 448kbps.
The saga continues - get this, they are using RDP. That pretty much explains all for me.
OK lemme get this straight... the office has ADSL on a VERY long line... syncing stable at 704 / 448 kbps.
And they are trying to run VPN / Terminal Server / Exchange ... all on the same line?
For any speedtests to be valid they need to disconnect all servers and services from the line and run a speed test from a single PC connected to the router. They can't run a speedtest and complain about the results whilst there's still VPN users and RDP sessions and mail coming in on the line.
Either that or they need to get something faster. If they can't afford a leased line then like Rizla said, they need to get another phone line or 3! And set up some kind of wan load balancing device.
To explain the effect of attenuation and SNR for you, briefly:
Attenuation is directly related to the length of the line, and shows how much the ADSL signal degrades (loses power) over the distance of the line. This should remain pretty static.
SNR is the Signal to Noise Ratio, or how much proper signal is on the line compared to the amount of noise. Ideally this would stay pretty static too, but when this goes down, it means there is more noise present on the line compared to what there was. Basically the wider the fluctuations in SNR, the more the end user will have problems with maintaining a stable connection.
So if your customer's line is staying up, there's no more BT can do. That's it. You probably run the risk of having charges levvied by BT if you push it back to them any more, since the line is stable. What exactly is your boss expecting BT to do? Suddenly make the line sync at 5Mb or something? Because it ain't gonna happen.
Besides, with VPN as Rizla also said, it's the upstream bandwidth that's the problem, not downstream. I wouldn't recommend going for a Max Premium (up to 832kbps upstream) regrade due to the length of the line, but they might be lucky and it works.
So they need to try testing the speeds directly from the office with NONE of their servers connected to the net, thereby ensuring nobody is using the VPN or RDP or exchange servers.
Out of interest, not looking for the exact answer here (not asking you to breach DPA!) but if you do have a way of looking at usage in GB or average kbps on the line, is that quite a lot? Dunno if you have access to RADIUS logs to see amount of traffic transferred in this session, but it might be worthwhile having a look, to see exactly how much they are using the line.